Provided by: erlang-manpages_25.3.2.8+dfsg-1ubuntu4_all 
NAME
calendar - Local and universal time, day of the week, date and time
conversions.
DESCRIPTION
This module provides computation of local and universal time, day of the week, and many
time conversion functions.
Time is local when it is adjusted in accordance with the current time zone and daylight
saving. Time is universal when it reflects the time at longitude zero, without any
adjustment for daylight saving. Universal Coordinated Time (UTC) time is also called
Greenwich Mean Time (GMT).
The time functions local_time/0 and universal_time/0 in this module both return date and
time. This is because separate functions for date and time can result in a date/time
combination that is displaced by 24 hours. This occurs if one of the functions is called
before midnight, and the other after midnight. This problem also applies to the Erlang
BIFs date/0 and time/0, and their use is strongly discouraged if a reliable date/time
stamp is required.
All dates conform to the Gregorian calendar. This calendar was introduced by Pope Gregory
XIII in 1582 and was used in all Catholic countries from this year. Protestant parts of
Germany and the Netherlands adopted it in 1698, England followed in 1752, and Russia in
1918 (the October revolution of 1917 took place in November according to the Gregorian
calendar).
The Gregorian calendar in this module is extended back to year 0. For a given date, the
gregorian days is the number of days up to and including the date specified. Similarly,
the gregorian seconds for a specified date and time is the number of seconds up to and
including the specified date and time.
For computing differences between epochs in time, use the functions counting gregorian
days or seconds. If epochs are specified as local time, they must be converted to
universal time to get the correct value of the elapsed time between epochs. Use of
function time_difference/2 is discouraged.
Different definitions exist for the week of the year. This module contains a week of the
year implementation conforming to the ISO 8601 standard. As the week number for a
specified date can fall on the previous, the current, or on the next year, it is important
to specify both the year and the week number. Functions iso_week_number/0 and
iso_week_number/1 return a tuple of the year and the week number.
DATA TYPES
datetime() = {date(), time()}
datetime1970() = {{year1970(), month(), day()}, time()}
date() = {year(), month(), day()}
year() = integer() >= 0
Year cannot be abbreviated. For example, 93 denotes year 93, not 1993. The valid
range depends on the underlying operating system. The date tuple must denote a
valid date.
year1970() = 1970..10000
month() = 1..12
day() = 1..31
time() = {hour(), minute(), second()}
hour() = 0..23
minute() = 0..59
second() = 0..59
daynum() = 1..7
ldom() = 28 | 29 | 30 | 31
yearweeknum() = {year(), weeknum()}
weeknum() = 1..53
rfc3339_time_unit() =
microsecond | millisecond | nanosecond | second | native
Note:
The native time unit was added to rfc3339_time_unit() in OTP 25.0.
EXPORTS
date_to_gregorian_days(Date) -> Days
date_to_gregorian_days(Year, Month, Day) -> Days
Types:
Date = date()
Year = year()
Month = month()
Day = day()
Computes the number of gregorian days starting with year 0 and ending at the
specified date.
datetime_to_gregorian_seconds(DateTime) -> Seconds
Types:
DateTime = datetime()
Seconds = integer() >= 0
Computes the number of gregorian seconds starting with year 0 and ending at the
specified date and time.
day_of_the_week(Date) -> daynum()
day_of_the_week(Year, Month, Day) -> daynum()
Types:
Date = date()
Year = year()
Month = month()
Day = day()
Computes the day of the week from the specified Year, Month, and Day. Returns the
day of the week as 1: Monday, 2: Tuesday, and so on.
gregorian_days_to_date(Days) -> date()
Types:
Days = integer() >= 0
Computes the date from the specified number of gregorian days.
gregorian_seconds_to_datetime(Seconds) -> datetime()
Types:
Seconds = integer() >= 0
Computes the date and time from the specified number of gregorian seconds.
is_leap_year(Year) -> boolean()
Types:
Year = year()
Checks if the specified year is a leap year.
iso_week_number() -> yearweeknum()
Returns tuple {Year, WeekNum} representing the ISO week number for the actual date.
To determine the actual date, use function local_time/0.
iso_week_number(Date) -> yearweeknum()
Types:
Date = date()
Returns tuple {Year, WeekNum} representing the ISO week number for the specified
date.
last_day_of_the_month(Year, Month) -> LastDay
Types:
Year = year()
Month = month()
LastDay = ldom()
Computes the number of days in a month.
local_time() -> datetime()
Returns the local time reported by the underlying operating system.
local_time_to_universal_time(DateTime1) -> DateTime2
Types:
DateTime1 = DateTime2 = datetime1970()
Converts from local time to Universal Coordinated Time (UTC). DateTime1 must refer
to a local date after Jan 1, 1970.
Warning:
This function is deprecated. Use local_time_to_universal_time_dst/1 instead, as it
gives a more correct and complete result. Especially for the period that does not
exist, as it is skipped during the switch to daylight saving time, this function
still returns a result.
local_time_to_universal_time_dst(DateTime1) -> [DateTime]
Types:
DateTime1 = DateTime = datetime1970()
Converts from local time to Universal Coordinated Time (UTC). DateTime1 must refer
to a local date after Jan 1, 1970.
The return value is a list of 0, 1, or 2 possible UTC times:
[]:
For a local {Date1, Time1} during the period that is skipped when switching to
daylight saving time, there is no corresponding UTC, as the local time is
illegal (it has never occured).
[DstDateTimeUTC, DateTimeUTC]:
For a local {Date1, Time1} during the period that is repeated when switching
from daylight saving time, two corresponding UTCs exist; one for the first
instance of the period when daylight saving time is still active, and one for
the second instance.
[DateTimeUTC]:
For all other local times only one corresponding UTC exists.
now_to_datetime(Now) -> datetime1970()
Types:
Now = erlang:timestamp()
Returns Universal Coordinated Time (UTC) converted from the return value from
erlang:timestamp/0.
now_to_local_time(Now) -> datetime1970()
Types:
Now = erlang:timestamp()
Returns local date and time converted from the return value from
erlang:timestamp/0.
now_to_universal_time(Now) -> datetime1970()
Types:
Now = erlang:timestamp()
Returns Universal Coordinated Time (UTC) converted from the return value from
erlang:timestamp/0.
rfc3339_to_system_time(DateTimeString) -> integer()
rfc3339_to_system_time(DateTimeString, Options) -> integer()
Types:
DateTimeString = rfc3339_string()
Options = [Option]
Option = {unit, rfc3339_time_unit()}
rfc3339_string() = [byte(), ...]
Converts an RFC 3339 timestamp into system time. The data format of RFC 3339
timestamps is described by RFC 3339. Starting from OTP 25.1, the minutes part of
the time zone is optional.
Valid option:
{unit, Unit}:
The time unit of the return value. The default is second.
1> calendar:rfc3339_to_system_time("2018-02-01T16:17:58+01:00").
1517498278
2> calendar:rfc3339_to_system_time("2018-02-01 15:18:02.088Z", [{unit, nanosecond}]).
1517498282088000000
seconds_to_daystime(Seconds) -> {Days, Time}
Types:
Seconds = Days = integer()
Time = time()
Converts a specified number of seconds into days, hours, minutes, and seconds. Time
is always non-negative, but Days is negative if argument Seconds is.
seconds_to_time(Seconds) -> time()
Types:
Seconds = secs_per_day()
secs_per_day() = 0..86400
Computes the time from the specified number of seconds. Seconds must be less than
the number of seconds per day (86400).
system_time_to_local_time(Time, TimeUnit) -> datetime()
Types:
Time = integer()
TimeUnit = erlang:time_unit()
Converts a specified system time into local date and time.
system_time_to_rfc3339(Time) -> DateTimeString
system_time_to_rfc3339(Time, Options) -> DateTimeString
Types:
Time = integer()
Options = [Option]
Option =
{offset, offset()} |
{time_designator, byte()} |
{unit, rfc3339_time_unit()}
DateTimeString = rfc3339_string()
offset() = [byte()] | (Time :: integer())
rfc3339_string() = [byte(), ...]
Converts a system time into an RFC 3339 timestamp. The data format of RFC 3339
timestamps is described by RFC 3339. The data format of offsets is also described
by RFC 3339.
Valid options:
{offset, Offset}:
The offset, either a string or an integer, to be included in the formatted
string. An empty string, which is the default, is interpreted as local time. A
non-empty string is included as is. The time unit of the integer is the same as
the one of Time.
{time_designator, Character}:
The character used as time designator, that is, the date and time separator.
The default is $T.
{unit, Unit}:
The time unit of Time. The default is second. If some other unit is given
(millisecond, microsecond, nanosecond, or native), the formatted string
includes a fraction of a second. The number of fractional second digits is
three, six, or nine depending on what time unit is chosen. For native three
fractional digits are included. Notice that trailing zeros are not removed from
the fraction.
1> calendar:system_time_to_rfc3339(erlang:system_time(second)).
"2018-04-23T14:56:28+02:00"
2> calendar:system_time_to_rfc3339(erlang:system_time(second), [{offset, "-02:00"}]).
"2018-04-23T10:56:52-02:00"
3> calendar:system_time_to_rfc3339(erlang:system_time(second), [{offset, -7200}]).
"2018-04-23T10:57:05-02:00"
4> calendar:system_time_to_rfc3339(erlang:system_time(millisecond), [{unit, millisecond}, {time_designator, $\s}, {offset, "Z"}]).
"2018-04-23 12:57:20.482Z"
system_time_to_universal_time(Time, TimeUnit) -> datetime()
Types:
Time = integer()
TimeUnit = erlang:time_unit()
Converts a specified system time into universal date and time.
time_difference(T1, T2) -> {Days, Time}
Types:
T1 = T2 = datetime()
Days = integer()
Time = time()
Returns the difference between two {Date, Time} tuples. T2 is to refer to an epoch
later than T1.
Warning:
This function is obsolete. Use the conversion functions for gregorian days and
seconds instead.
time_to_seconds(Time) -> secs_per_day()
Types:
Time = time()
secs_per_day() = 0..86400
Returns the number of seconds since midnight up to the specified time.
universal_time() -> datetime()
Returns the Universal Coordinated Time (UTC) reported by the underlying operating
system. Returns local time if universal time is unavailable.
universal_time_to_local_time(DateTime) -> datetime()
Types:
DateTime = datetime1970()
Converts from Universal Coordinated Time (UTC) to local time. DateTime must refer
to a date after Jan 1, 1970.
valid_date(Date) -> boolean()
valid_date(Year, Month, Day) -> boolean()
Types:
Date = date()
Year = Month = Day = integer()
This function checks if a date is a valid.
LEAP YEARS
The notion that every fourth year is a leap year is not completely true. By the Gregorian
rule, a year Y is a leap year if one of the following rules is valid:
* Y is divisible by 4, but not by 100.
* Y is divisible by 400.
Hence, 1996 is a leap year, 1900 is not, but 2000 is.
DATE AND TIME SOURCE
Local time is obtained from the Erlang BIF localtime/0. Universal time is computed from
the BIF universaltime/0.
The following apply:
* There are 86400 seconds in a day.
* There are 365 days in an ordinary year.
* There are 366 days in a leap year.
* There are 1461 days in a 4 year period.
* There are 36524 days in a 100 year period.
* There are 146097 days in a 400 year period.
* There are 719528 days between Jan 1, 0 and Jan 1, 1970.