Provided by: tcl8.6-doc_8.6.12+dfsg-1build1_all 
NAME
clock - Obtain and manipulate dates and times
SYNOPSIS
package require Tcl 8.5
clock add timeVal ?count unit...? ?-option value?
clock clicks ?-option?
clock format timeVal ?-option value...?
clock microseconds
clock milliseconds
clock scan inputString ?-option value...?
clock seconds
_________________________________________________________________________________________________
DESCRIPTION
The clock command performs several operations that obtain and manipulate values that
represent times. The command supports several subcommands that determine what action is
carried out by the command.
clock add timeVal ?count unit...? ?-option value?
Adds a (possibly negative) offset to a time that is expressed as an integer number
of seconds. See CLOCK ARITHMETIC for a full description.
clock clicks ?-option?
If no -option argument is supplied, returns a high-resolution time value as a
system-dependent integer value. The unit of the value is system-dependent but
should be the highest resolution clock available on the system such as a CPU cycle
counter. See HIGH RESOLUTION TIMERS for a full description.
If the -option argument is -milliseconds, then the command is synonymous with clock
milliseconds (see below). This usage is obsolete, and clock milliseconds is to be
considered the preferred way of obtaining a count of milliseconds.
If the -option argument is -microseconds, then the command is synonymous with clock
microseconds (see below). This usage is obsolete, and clock microseconds is to be
considered the preferred way of obtaining a count of microseconds.
clock format timeVal ?-option value...?
Formats a time that is expressed as an integer number of seconds into a format
intended for consumption by users or external programs. See FORMATTING TIMES for a
full description.
clock microseconds
Returns the current time as an integer number of microseconds. See HIGH RESOLUTION
TIMERS for a full description.
clock milliseconds
Returns the current time as an integer number of milliseconds. See HIGH RESOLUTION
TIMERS for a full description.
clock scan inputString ?-option value...?
Scans a time that is expressed as a character string and produces an integer number
of seconds. See SCANNING TIMES for a full description.
clock seconds
Returns the current time as an integer number of seconds.
PARAMETERS
count An integer representing a count of some unit of time. See CLOCK ARITHMETIC for the
details.
timeVal
An integer value passed to the clock command that represents an absolute time as a
number of seconds from the epoch time of 1 January 1970, 00:00 UTC. Note that the
count of seconds does not include any leap seconds; seconds are counted as if each
UTC day has exactly 86400 seconds. Tcl responds to leap seconds by speeding or
slowing its clock by a tiny fraction for some minutes until it is back in sync with
UTC; its data model does not represent minutes that have 59 or 61 seconds.
unit One of the words, seconds, minutes, hours, days, weeks, months, or years, or any
unique prefix of such a word. Used in conjunction with count to identify an
interval of time, for example, 3 seconds or 1 year.
OPTIONS
-base time
Specifies that any relative times present in a clock scan command are to be given
relative to time. time must be expressed as a count of nominal seconds from the
epoch time of 1 January 1970, 00:00 UTC.
-format format
Specifies the desired output format for clock format or the expected input format
for clock scan. The format string consists of any number of characters other than
the per-cent sign (“%”) interspersed with any number of format groups, which are
two-character sequences beginning with the per-cent sign. The permissible format
groups, and their interpretation, are described under FORMAT GROUPS.
On clock format, the default format is
%a %b %d %H:%M:%S %Z %Y
On clock scan, the lack of a -format option indicates that a “free format scan” is
requested; see FREE FORM SCAN for a description of what happens.
-gmt boolean
If boolean is true, specifies that a time specified to clock add, clock format or
clock scan should be processed in UTC. If boolean is false, the processing
defaults to the local time zone. This usage is obsolete; the correct current usage
is to specify the UTC time zone with “-timezone :UTC” or any of the equivalent ways
to specify it.
-locale localeName
Specifies that locale-dependent scanning and formatting (and date arithmetic for
dates preceding the adoption of the Gregorian calendar) is to be done in the locale
identified by localeName. The locale name may be any of the locales acceptable to
the msgcat package, or it may be the special name system, which represents the
current locale of the process, or the null string, which represents Tcl's default
locale.
The effect of locale on scanning and formatting is discussed in the descriptions of
the individual format groups under FORMAT GROUPS. The effect of locale on clock
arithmetic is discussed under CLOCK ARITHMETIC.
-timezone zoneName
Specifies that clock arithmetic, formatting, and scanning are to be done according
to the rules for the time zone specified by zoneName. The permissible values, and
their interpretation, are discussed under TIME ZONES. On subcommands that expect a
-timezone argument, the default is to use the current time zone. The current time
zone is determined, in order of preference, by:
[1] the environment variable TCL_TZ.
[2] the environment variable TZ.
[3] on Windows systems, the time zone settings from the Control Panel.
If none of these is present, the C localtime and mktime functions are used to attempt to
convert times between local and Greenwich. On 32-bit systems, this approach is likely to
have bugs, particularly for times that lie outside the window (approximately the years
1902 to 2037) that can be represented in a 32-bit integer.
CLOCK ARITHMETIC
The clock add command performs clock arithmetic on a value (expressed as nominal seconds
from the epoch time of 1 January 1970, 00:00 UTC) given as its first argument. The
remaining arguments (other than the possible -timezone, -locale and -gmt options) are
integers and keywords in alternation, where the keywords are chosen from seconds, minutes,
hours, days, weeks, months, or years, or any unique prefix of such a word.
Addition of seconds, minutes and hours is fairly straightforward; the given time increment
(times sixty for minutes, or 3600 for hours) is simply added to the timeVal given to the
clock add command. The result is interpreted as a nominal number of seconds from the
Epoch.
Surprising results may be obtained when crossing a point at which a leap second is
inserted or removed; the clock add command simply ignores leap seconds and therefore
assumes that times come in sequence, 23:59:58, 23:59:59, 00:00:00. (This assumption is
handled by the fact that Tcl's model of time reacts to leap seconds by speeding or slowing
the clock by a minuscule amount until Tcl's time is back in step with the world.
The fact that adding and subtracting hours is defined in terms of absolute time means that
it will add fixed amounts of time in time zones that observe summer time (Daylight Saving
Time). For example, the following code sets the value of x to 04:00:00 because the clock
has changed in the interval in question.
set s [clock scan {2004-10-30 05:00:00} \
-format {%Y-%m-%d %H:%M:%S} \
-timezone :America/New_York]
set a [clock add $s 24 hours -timezone :America/New_York]
set x [clock format $a \
-format {%H:%M:%S} -timezone :America/New_York]
Adding and subtracting days and weeks is accomplished by converting the given time to a
calendar day and time of day in the appropriate time zone and locale. The requisite
number of days (weeks are converted to days by multiplying by seven) is added to the
calendar day, and the date and time are then converted back to a count of seconds from the
epoch time.
Adding and subtracting a given number of days across the point that the time changes at
the start or end of summer time (Daylight Saving Time) results in the same local time on
the day in question. For instance, the following code sets the value of x to 05:00:00.
set s [clock scan {2004-10-30 05:00:00} \
-format {%Y-%m-%d %H:%M:%S} \
-timezone :America/New_York]
set a [clock add $s 1 day -timezone :America/New_York]
set x [clock format $a \
-format {%H:%M:%S} -timezone :America/New_York]
In cases of ambiguity, where the same local time happens twice on the same day, the
earlier time is used. In cases where the conversion yields an impossible time (for
instance, 02:30 during the Spring Daylight Saving Time change using US rules), the time is
converted as if the clock had not changed. Thus, the following code will set the value of
x to 03:30:00.
set s [clock scan {2004-04-03 02:30:00} \
-format {%Y-%m-%d %H:%M:%S} \
-timezone :America/New_York]
set a [clock add $s 1 day -timezone :America/New_York]
set x [clock format $a \
-format {%H:%M:%S} -timezone :America/New_York]
Adding a given number of days or weeks works correctly across the conversion between the
Julian and Gregorian calendars; the omitted days are skipped. The following code sets z
to 1752-09-14.
set x [clock scan 1752-09-02 -format %Y-%m-%d -locale en_US]
set y [clock add $x 1 day -locale en_US]
set z [clock format $y -format %Y-%m-%d -locale en_US]
In the bizarre case that adding the given number of days yields a date that does not exist
because it falls within the dropped days of the Julian-to-Gregorian conversion, the date
is converted as if it was on the Julian calendar.
Adding a number of months, or a number of years, is similar; it converts the given time to
a calendar date and time of day. It then adds the requisite number of months or years,
and reconverts the resulting date and time of day to an absolute time.
If the resulting date is impossible because the month has too few days (for example, when
adding 1 month to 31 January), the last day of the month is substituted. Thus, adding 1
month to 31 January will result in 28 February in a common year or 29 February in a leap
year.
The rules for handling anomalies relating to summer time and to the Gregorian calendar are
the same when adding/subtracting months and years as they are when adding/subtracting days
and weeks.
If multiple count unit pairs are present on the command, they are evaluated consecutively,
from left to right.
HIGH RESOLUTION TIMERS
Most of the subcommands supported by the clock command deal with times represented as a
count of seconds from the epoch time, and this is the representation that clock seconds
returns. There are three exceptions, which are all intended for use where higher-
resolution times are required. clock milliseconds returns the count of milliseconds from
the epoch time, and clock microseconds returns the count of microseconds from the epoch
time. In addition, there is a clock clicks command that returns a platform-dependent high-
resolution timer. Unlike clock seconds and clock milliseconds, the value of clock clicks
is not guaranteed to be tied to any fixed epoch; it is simply intended to be the most
precise interval timer available, and is intended only for relative timing studies such as
benchmarks.
FORMATTING TIMES
The clock format command produces times for display to a user or writing to an external
medium. The command accepts times that are expressed in seconds from the epoch time of 1
January 1970, 00:00 UTC, as returned by clock seconds, clock scan, clock add, file atime
or file mtime.
If a -format option is present, the following argument is a string that specifies how the
date and time are to be formatted. The string consists of any number of characters other
than the per-cent sign (“%”) interspersed with any number of format groups, which are two-
character sequences beginning with the per-cent sign. The permissible format groups, and
their interpretation, are described under FORMAT GROUPS.
If a -timezone option is present, the following argument is a string that specifies the
time zone in which the date and time are to be formatted. As an alternative to “-timezone
:UTC”, the obsolete usage “-gmt true” may be used. See TIME ZONES for the permissible
variants for the time zone.
If a -locale option is present, the following argument is a string that specifies the
locale in which the time is to be formatted, in the same format that is used for the
msgcat package. Note that the default, if -locale is not specified, is the root locale {}
rather than the current locale. The current locale may be obtained by using -locale
current. In addition, some platforms support a system locale that reflects the user's
current choices. For instance, on Windows, the format that the user has selected from
dates and times in the Control Panel can be obtained by using the system locale. On
platforms that do not define a user selection of date and time formats separate from
LC_TIME, -locale system is synonymous with -locale current.
SCANNING TIMES
The clock scan command accepts times that are formatted as strings and converts them to
counts of seconds from the epoch time of 1 January 1970, 00:00 UTC. It normally takes a
-format option that is followed by a string describing the expected format of the input.
(See FREE FORM SCAN for the effect of clock scan without such an argument.) The string
consists of any number of characters other than the per-cent sign (“%”), interspersed with
any number of format groups, which are two-character sequences beginning with the per-cent
sign. The permissible format groups, and their interpretation, are described under FORMAT
GROUPS.
If a -timezone option is present, the following argument is a string that specifies the
time zone in which the date and time are to be interpreted. As an alternative to
-timezone :UTC, the obsolete usage -gmt true may be used. See TIME ZONES for the
permissible variants for the time zone.
If a -locale option is present, the following argument is a string that specifies the
locale in which the time is to be interpreted, in the same format that is used for the
msgcat package. Note that the default, if -locale is not specified, is the root locale {}
rather than the current locale. The current locale may be obtained by using -locale
current. In addition, some platforms support a system locale that reflects the user's
current choices. For instance, on Windows, the format that the user has selected from
dates and times in the Control Panel can be obtained by using the system locale. On
platforms that do not define a user selection of date and time formats separate from
LC_TIME, -locale system is synonymous with -locale current.
If a -base option is present, the following argument is a time (expressed in seconds from
the epoch time) that is used as a base time for interpreting relative times. If no -base
option is present, the base time is the current time.
Scanning of times in fixed format works by determining three things: the date, the time of
day, and the time zone. These three are then combined into a point in time, which is
returned as the number of seconds from the epoch.
Before scanning begins, the format string is preprocessed to replace %c, %Ec, %x, %Ex, %X.
%Ex, %r, %R, %T, %D, %EY and %+ format groups with counterparts that are appropriate to
the current locale and contain none of the above groups. For instance, %D will (in the
en_US locale) be replaced with %m/%d/%Y.
The date is determined according to the fields that are present in the preprocessed format
string. In order of preference:
[1] If the string contains a %s format group, representing seconds from the epoch, that
group is used to determine the date.
[2] If the string contains a %J format group, representing the Julian Day Number, that
group is used to determine the date.
[3] If the string contains a complete set of format groups specifying century, year,
month, and day of month; century, year, and day of year; or ISO8601 fiscal year,
week of year, and day of week; those groups are combined and used to determine the
date. If more than one complete set is present, the one at the rightmost position
in the string is used.
[4] If the string lacks a century but contains a set of format groups specifying year
of century, month and day of month; year of century and day of year; or two-digit
ISO8601 fiscal year, week of year, and day of week; those groups are combined and
used to determine the date. If more than one complete set is present, the one at
the rightmost position in the string is used. The year is presumed to lie in the
range 1938 to 2037 inclusive.
[5] If the string entirely lacks any specification for the year (or contains the year
only on the locale's alternative calendar) and contains a set of format groups
specifying month and day of month, day of year, or week of year and day of week,
those groups are combined and used to determine the date. If more than one
complete set is present, the one at the rightmost position in the string is used.
The year is determined by interpreting the base time in the given time zone.
[6] If the string contains none of the above sets, but has a day of the month or day of
the week, the day of the month or day of the week are used to determine the date by
interpreting the base time in the given time zone and returning the given day of
the current week or month. (The week runs from Monday to Sunday, ISO8601-fashion.)
If both day of month and day of week are present, the day of the month takes
priority.
[7] If none of the above rules results in a usable date, the date of the base time in
the given time zone is used.
The time is also determined according to the fields that are present in the preprocessed
format string. In order of preference:
[1] If the string contains a %s format group, representing seconds from the epoch, that
group determines the time of day.
[2] If the string contains either an hour on the 24-hour clock or an hour on the
12-hour clock plus an AM/PM indicator, that hour determines the hour of the day.
If the string further contains a group specifying the minute of the hour, that
group combines with the hour. If the string further contains a group specifying
the second of the minute, that group combines with the hour and minute.
[3] If the string contains neither a %s format group nor a group specifying the hour of
the day, then midnight (00:00, the start of the given date) is used. The time zone
is determined by either the -timezone or -gmt options, or by using the current time
zone.
If a format string lacks a %z or %Z format group, it is possible for the time to be
ambiguous because it appears twice in the same day, once without and once with Daylight
Saving Time. If this situation occurs, the first occurrence of the time is chosen. (For
this reason, it is wise to have the input string contain the time zone when converting
local times. This caveat does not apply to UTC times.)
If the interpretation of the groups yields an impossible time because a field is out of
range, enough of that field's unit will be added to or subtracted from the time to bring
it in range. Thus, if attempting to scan or format day 0 of the month, one day will be
subtracted from day 1 of the month, yielding the last day of the previous month.
If the interpretation of the groups yields an impossible time because a Daylight Saving
Time change skips over that time, or an ambiguous time because a Daylight Saving Time
change skips back so that the clock observes the given time twice, and no time zone
specifier (%z or %Z) is present in the format, the time is interpreted as if the clock had
not changed.
FORMAT GROUPS
The following format groups are recognized by the clock scan and clock format commands.
%a On output, produces an abbreviation (e.g., Mon) for the day of the week in the
given locale. On input, matches the name of the day of the week in the given
locale (in either abbreviated or full form, or any unique prefix of either form).
%A On output, produces the full name (e.g., Monday) of the day of the week in the
given locale. On input, matches the name of the day of the week in the given
locale (in either abbreviated or full form, or any unique prefix of either form).
%b On output, produces an abbreviation (e.g., Jan) for the name of the month in the
given locale. On input, matches the name of the month in the given locale (in
either abbreviated or full form, or any unique prefix of either form).
%B On output, produces the full name (e.g., January) of the month in the given locale.
On input, matches the name of the month in the given locale (in either abbreviated
or full form, or any unique prefix of either form).
%c On output, produces a localized representation of date and time of day; the
localized representation is expected to use the Gregorian calendar. On input,
matches whatever %c produces.
%C On output, produces the number of the century in Indo-Arabic numerals. On input,
matches one or two digits, possibly with leading whitespace, that are expected to
be the number of the century.
%d On output, produces the number of the day of the month, as two decimal digits. On
input, matches one or two digits, possibly with leading whitespace, that are
expected to be the number of the day of the month.
%D This format group is synonymous with %m/%d/%Y. It should be used only in
exchanging data within the en_US locale, since other locales typically do not use
this order for the fields of the date.
%e On output, produces the number of the day of the month, as one or two decimal
digits (with a leading blank for one-digit dates). On input, matches one or two
digits, possibly with leading whitespace, that are expected to be the number of the
day of the month.
%Ec On output, produces a locale-dependent representation of the date and time of day
in the locale's alternative calendar. On input, matches whatever %Ec produces.
The locale's alternative calendar need not be the Gregorian calendar.
%EC On output, produces a locale-dependent name of an era in the locale's alternative
calendar. On input, matches the name of the era or any unique prefix.
%EE On output, produces the string B.C.E. or C.E., or a string of the same meaning in
the locale, to indicate whether %Y refers to years before or after Year 1 of the
Common Era. On input, accepts the string B.C.E., B.C., C.E., A.D., or the
abbreviation appropriate to the current locale, and uses it to fix whether %Y
refers to years before or after Year 1 of the Common Era.
%Ex On output, produces a locale-dependent representation of the date in the locale's
alternative calendar. On input, matches whatever %Ex produces. The locale's
alternative calendar need not be the Gregorian calendar.
%EX On output, produces a locale-dependent representation of the time of day in the
locale's alternative numerals. On input, matches whatever %EX produces.
%Ey On output, produces a locale-dependent number of the year of the era in the
locale's alternative calendar and numerals. On input, matches such a number.
%EY On output, produces a representation of the year in the locale's alternative
calendar and numerals. On input, matches what %EY produces. Often synonymous with
%EC%Ey.
%g On output, produces a two-digit year number suitable for use with the week-based
ISO8601 calendar; that is, the year number corresponds to the week number produced
by %V. On input, accepts such a two-digit year number, possibly with leading
whitespace.
%G On output, produces a four-digit year number suitable for use with the week-based
ISO8601 calendar; that is, the year number corresponds to the week number produced
by %V. On input, accepts such a four-digit year number, possibly with leading
whitespace.
%h This format group is synonymous with %b.
%H On output, produces a two-digit number giving the hour of the day (00-23) on a
24-hour clock. On input, accepts such a number.
%I On output, produces a two-digit number giving the hour of the day (12-11) on a
12-hour clock. On input, accepts such a number.
%j On output, produces a three-digit number giving the day of the year (001-366). On
input, accepts such a number.
%J On output, produces a string of digits giving the Julian Day Number. On input,
accepts a string of digits and interprets it as a Julian Day Number. The Julian
Day Number is a count of the number of calendar days that have elapsed since 1
January, 4713 BCE of the proleptic Julian calendar. The epoch time of 1 January
1970 corresponds to Julian Day Number 2440588.
%k On output, produces a one- or two-digit number giving the hour of the day (0-23) on
a 24-hour clock. On input, accepts such a number.
%l On output, produces a one- or two-digit number giving the hour of the day (12-11)
on a 12-hour clock. On input, accepts such a number.
%m On output, produces the number of the month (01-12) with exactly two digits. On
input, accepts two digits and interprets them as the number of the month.
%M On output, produces the number of the minute of the hour (00-59) with exactly two
digits. On input, accepts two digits and interprets them as the number of the
minute of the hour.
%N On output, produces the number of the month (1-12) with one or two digits, and a
leading blank for one-digit dates. On input, accepts one or two digits, possibly
with leading whitespace, and interprets them as the number of the month.
%Od, %Oe, %OH, %OI, %Ok, %Ol, %Om, %OM, %OS, %Ou, %Ow, %Oy
All of these format groups are synonymous with their counterparts without the “O”,
except that the string is produced and parsed in the locale-dependent alternative
numerals.
%p On output, produces an indicator for the part of the day, AM or PM, appropriate to
the given locale. If the script of the given locale supports multiple letterforms,
lowercase is preferred. On input, matches the representation AM or PM in the given
locale, in either case.
%P On output, produces an indicator for the part of the day, am or pm, appropriate to
the given locale. If the script of the given locale supports multiple letterforms,
uppercase is preferred. On input, matches the representation AM or PM in the given
locale, in either case.
%Q This format group is reserved for internal use within the Tcl library.
%r On output, produces a locale-dependent time of day representation on a 12-hour
clock. On input, accepts whatever %r produces.
%R On output, the time in 24-hour notation (%H:%M). For a version including the
seconds, see %T below. On input, accepts whatever %R produces.
%s On output, simply formats the timeVal argument as a decimal integer and inserts it
into the output string. On input, accepts a decimal integer and uses is as the
time value without any further processing. Since %s uniquely determines a point in
time, it overrides all other input formats.
%S On output, produces a two-digit number of the second of the minute (00-59). On
input, accepts two digits and uses them as the second of the minute.
%t On output, produces a TAB character. On input, matches a TAB character.
%T Synonymous with %H:%M:%S.
%u On output, produces the number of the day of the week (1→Monday, 7→Sunday). On
input, accepts a single digit and interprets it as the day of the week. Sunday may
be either 0 or 7.
%U On output, produces the ordinal number of the week of the year (00-53). The first
Sunday of the year is the first day of week 01. On input accepts two digits which
are otherwise ignored. This format group is never used in determining an input
date. This interpretation of the week of the year was once common in US banking
but is now largely obsolete. See %V for the ISO8601 week number.
%V On output, produces the number of the ISO8601 week as a two digit number (01-53).
Week 01 is the week containing January 4; or the first week of the year containing
at least 4 days; or the week containing the first Thursday of the year (the three
statements are equivalent). Each week begins on a Monday. On input, accepts the
ISO8601 week number.
%w On output, produces the ordinal number of the day of the week (Sunday==0;
Saturday==6). On input, accepts a single digit and interprets it as the day of the
week; Sunday may be represented as either 0 or 7. Note that %w is not the ISO8601
weekday number, which is produced and accepted by %u.
%W On output, produces a week number (00-53) within the year; week 01 begins on the
first Monday of the year. On input, accepts two digits, which are otherwise
ignored. This format group is never used in determining an input date. It is not
the ISO8601 week number; that week is produced and accepted by %V.
%x On output, produces the date in a locale-dependent representation. On input,
accepts whatever %x produces and is used to determine calendar date.
%X On output, produces the time of day in a locale-dependent representation. On input,
accepts whatever %X produces and is used to determine time of day.
%y On output, produces the two-digit year of the century. On input, accepts two
digits, and is used to determine calendar date. The date is presumed to lie between
1938 and 2037 inclusive. Note that %y does not yield a year appropriate for use
with the ISO8601 week number %V; programs should use %g for that purpose.
%Y On output, produces the four-digit calendar year. On input, accepts four digits and
may be used to determine calendar date. Note that %Y does not yield a year
appropriate for use with the ISO8601 week number %V; programs should use %G for
that purpose.
%z On output, produces the current time zone, expressed in hours and minutes east
(+hhmm) or west (-hhmm) of Greenwich. On input, accepts a time zone specifier (see
TIME ZONES below) that will be used to determine the time zone.
%Z On output, produces the current time zone's name, possibly translated to the given
locale. On input, accepts a time zone specifier (see TIME ZONES below) that will be
used to determine the time zone. This option should, in general, be used on input
only when parsing RFC822 dates. Other uses are fraught with ambiguity; for
instance, the string BST may represent British Summer Time or Brazilian Standard
Time. It is recommended that date/time strings for use by computers use numeric
time zones instead.
%% On output, produces a literal “%” character. On input, matches a literal “%”
character.
%+ Synonymous with “%a %b %e %H:%M:%S %Z %Y”.
TIME ZONES
When the clock command is processing a local time, it has several possible sources for the
time zone to use. In order of preference, they are:
[1] A time zone specified inside a string being parsed and matched by a %z or %Z format
group.
[2] A time zone specified with the -timezone option to the clock command (or,
equivalently, by -gmt 1).
[3] A time zone specified in an environment variable TCL_TZ.
[4] A time zone specified in an environment variable TZ.
[5] The local time zone from the Control Panel on Windows systems.
[6] The C library's idea of the local time zone, as defined by the mktime and localtime
functions.
In case [1] only, the string is tested to see if it is one of the strings:
gmt ut utc bst wet wat at
nft nst ndt ast adt est edt
cst cdt mst mdt pst pdt yst
ydt hst hdt cat ahst nt idlw
cet cest met mewt mest swt sst
eet eest bt it zp4 zp5 ist
zp6 wast wadt jt cct jst cast
cadt east eadt gst nzt nzst nzdt
idle
If it is a string in the above list, it designates a known time zone, and is interpreted
as such.
For time zones in case [1] that do not match any of the above strings, and always for
cases [2]-[6], the following rules apply.
If the time zone begins with a colon, it is one of a standardized list of names like
:America/New_York that give the rules for various locales. A complete list of the
location names is too lengthy to be listed here. On most Tcl installations, the
definitions of the locations are to be found in named files in the directory
“/no_backup/tools/lib/tcl8.5/clock/tzdata”. On some Unix systems, these files are
omitted, and the definitions are instead obtained from system files in
“/usr/share/zoneinfo”, “/usr/share/lib/zoneinfo” or “/usr/local/etc/zoneinfo”. As a
special case, the name :localtime refers to the local time zone as defined by the C
library.
A time zone string consisting of a plus or minus sign followed by four or six decimal
digits is interpreted as an offset in hours, minutes, and seconds (if six digits are
present) from UTC. The plus sign denotes a sign east of Greenwich; the minus sign one
west of Greenwich.
A time zone string conforming to the Posix specification of the TZ environment variable
will be recognized. The specification may be found at
https://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html.
If the Posix time zone string contains a DST (Daylight Savings Time) part, but doesn't
contain a rule stating when DST starts or ends, then default rules are used. For Timezones
with an offset between 0 and +12, the current European/Russian rules are used, otherwise
the current US rules are used. In Europe (offset +0 to +2) the switch to summertime is
done each last Sunday in March at 1:00 GMT, and the switch back is each last Sunday in
October at 2:00 GMT. In Russia (offset +3 to +12), the switch dates are the same, only the
switch to summertime is at 2:00 local time, and the switch back is at 3:00 local time in
all time zones. The US switch to summertime takes place each second Sunday in March at
2:00 local time, and the switch back is each first Sunday in November at 3:00 local time.
These default rules mean that in all European, Russian and US (or compatible) time zones,
DST calculations will be correct for dates in 2007 and later, unless in the future the
rules change again.
Any other time zone string is processed by prefixing a colon and attempting to use it as a
location name, as above.
LOCALIZATION
Developers wishing to localize the date and time formatting and parsing are referred to
https://tip.tcl-lang.org/173 for a specification.
FREE FORM SCAN
If the clock scan command is invoked without a -format option, then it requests a free-
form scan. This form of scan is deprecated. The reason for the deprecation is that there
are too many ambiguities. (Does the string “2000” represent a year, a time of day, or a
quantity?) No set of rules for interpreting free-form dates and times has been found to
give unsurprising results in all cases.
If free-form scan is used, only the -base and -gmt options are accepted. The -timezone
and -locale options will result in an error if -format is not supplied.
For the benefit of users who need to understand legacy code that uses free-form scan, the
documentation for how free-form scan interprets a string is included here:
If only a time is specified, the current date is assumed. If the inputString does not
contain a time zone mnemonic, the local time zone is assumed, unless the -gmt argument is
true, in which case the clock value is calculated assuming that the specified time is
relative to Greenwich Mean Time. -gmt, if specified, affects only the computed time
value; it does not impact the interpretation of -base.
If the -base flag is specified, the next argument should contain an integer clock value.
Only the date in this value is used, not the time. This is useful for determining the
time on a specific day or doing other date-relative conversions.
The inputString argument consists of zero or more specifications of the following form:
time A time of day, which is of the form: hh?:mm?:ss?? ?meridian? ?zone? or hhmm
?meridian? ?zone? If no meridian is specified, hh is interpreted on a 24-hour
clock.
date A specific month and day with optional year. The acceptable formats are
“mm/dd?/yy?”, “monthname dd?, yy?”, “day, dd monthname ?yy?”, “dd monthname yy”,
“?CC?yymmdd”, and “dd-monthname-?CC?yy”. The default year is the current year. If
the year is less than 100, we treat the years 00-68 as 2000-2068 and the years
69-99 as 1969-1999. Not all platforms can represent the years 38-70, so an error
may result if these years are used.
ISO 8601 point-in-time
An ISO 8601 point-in-time specification, such as “CCyymmddThhmmss,” where T is the
literal “T”, “CCyymmdd hhmmss”, “CCyymmddThh:mm:ss”, or “CCyy-mm-ddThh:mm:ss.”
Note that only these four formats are accepted. The command does not accept the
full range of point-in-time specifications specified in ISO8601. Other formats can
be recognized by giving an explicit -format option to the clock scan command.
relative time
A specification relative to the current time. The format is number unit.
Acceptable units are year, fortnight, month, week, day, hour, minute (or min), and
second (or sec). The unit can be specified as a singular or plural, as in 3 weeks.
These modifiers may also be specified: tomorrow, yesterday, today, now, last, this,
next, ago.
The actual date is calculated according to the following steps.
First, any absolute date and/or time is processed and converted. Using that time as the
base, day-of-week specifications are added. Next, relative specifications are used. If a
date or day is specified, and no absolute or relative time is given, midnight is used.
Finally, a correction is applied so that the correct hour of the day is produced after
allowing for daylight savings time differences and the correct date is given when going
from the end of a long month to a short month.
SEE ALSO
msgcat(3tcl)
KEYWORDS
clock, date, time
COPYRIGHT
Copyright © 2004 Kevin B. Kenny <kennykb@acm.org>. All rights reserved.