Provided by: libdate-manip-perl_6.60-1_all 
NAME
Date::Manip::Calc - describes date calculations
SYNOPSIS
Two objects (both of which are either Date::Manip::Date or Date::Manip::Delta objects) may
be used to creates a third object based on those two.
$delta = $date->calc($date2 [,$subtract] [,$mode]);
$date2 = $date->calc($delta [,$subtract]);
$date2 = $delta->calc($date1 [,$subtract]);
$delta3 = $delta1->calc($delta2 [,$subtract] [,$no_normalize]);
DESCRIPTION
This document describes the different types of calculations that can be done using dates
and deltas. Date calculations are much more complicated than they initially appear, so
this document is fairly large.
The complication in date calculations is due to the fact that it is impossible to express
some parts of a delta as an exact length. Some examples will illustrate this:
As an example, let's take two dates and determine how much time elapsed between them:
Nov 3 2016 11:00:00
Dec 5 2016 12:00:00
Elapsed time: 770 hours
There are several ways to describe the time that elapsed. The first way is to give the
difference exactly. This is the exact delta.
An exact delta is always described in terms of hours, minutes, and seconds.
The problem with this is that we don't think in terms of exact deltas. We think in terms
which cannot be expressed exactly.
For example, most people would look at those two dates and think:
Percieved: 1 month, 2 days, 1 hour
But the two dates:
Feb 3 2016 11:00:00
Mar 5 2016 12:00:00
Elapsed time: 745 hours
Perceived: 1 month, 2 days, 1 hour
Some fields in a delta do not have an exact length. A year is usually 365 days long, but
sometimes it is 366. A month might be 28, 29, 30, or 31 days long.
Perhaps the most unexpected difficulty is that days are not of constant length. Most
people would define a day as 24 hours, but when you take daylight saving time into account
that definition produces unexpected results. The following calculation illustrates this:
Nov 5, 2011 02:30 EDT
+ 24 hour
Result: Nov 6, 2011 01:30 EST
This immediately causes most people to redefine a day as the amount of time between the
same wallclock time. For example, the amount of time between noon one day and noon the
next (regardless of daylight saving time changes).
This definition doesn't work either. For example:
Mar 12, 2011 02:30 EST
+ 1 day (same time next day)
Result: Mar 13 02:30 EST
But that date does not exist! Neither does:
Result: Mar 13 02:30 EDT
An alternate calculation could be:
Nov 5, 2011 01:30 EDT
+ 1 day (same time next day)
Result: Nov 6, 01:30 EDT
Result: Nov 6, 01:30 EST
Both of those results exist. Which result did you mean? The first one is probably
correct (since it is 24 hours later), but an hour later, you will have the same clock time
again.
So, the same time next day definition doesn't work at all for some dates (during a 'spring
forward' type daylight saving time transition) and is ambiguous for others (during a 'fall
back' type daylight saving time transition).
Calculations involving exact deltas are unambiguous in all cases.
A second class of delta is called a semi-exact delta, and these add days (and weeks) to
the delta, and treats days as a "same time next day" at all times except the two cases
where the resulting date falls in the period where a daylight saving time transition is
occuring. Then it falls back to the 24 hour definition.
A final class of delta is an approximate delta which includes all of the fields (years and
months). This allows Date::Manip to handle deltas in a way that is consistent with how
most people perceive the elapsed time. It should be noted that there is some uncertaintly
there as not everyone's definition of how a delta is perceived is the same, but in
general, they should be closer to what most people think of.
TYPES OF CALCULATIONS
This document describes the different types of calculations. Calculations involve two
types of Date::Manip objects: dates and deltas. These are described in the
Date::Manip::Date and Date::Manip::Delta manuals respectively.
Two objects (two dates, two deltas, or one of each) are used. In all cases, if a second
object is not passed in, undef is returned.
There are 3 types of calculations:
Date/Date calculations
A calculation involving 2 dates is used to determine the amount of time (the delta)
between them.
$delta = $date1->calc($date2 [,$subtract] [,$mode]);
Two dates can be worked with and a delta will be produced which is the amount of time
between the two dates.
$date1 and $date2 are Date::Manip::Date objects with valid dates. The
Date::Manip::Delta object returned is the amount of time between them. If $subtract is
not passed in (or is 0), the delta produced is:
DELTA = DATE2 - DATE1
If $subtract is non-zero, the delta produced is:
DELTA = DATE1 - DATE2
The $subtract argument has special importance when doing approximate calculations, and
this is described below.
If either date is invalid, a delta object will be returned which has an error
associated with it.
The $mode argument describes the type of delta that is produced and is described below
in "MODE".
Date/Delta calculations
Date/delta calculations can be performed using either a Date::Manip::Date or
Date::Manip::Delta object as the primary object:
$date2 = $date1->calc($delta [,$subtract]);
$date2 = $delta->calc($date1 [,$subtract]);
A date and delta can be combined to yield a date that is the given amount of time
before or after it.
$date1 and $delta are Date::Manip::Date and Date::Manip::Delta objects respectively. A
new Date::Manip::Date object is produced. If either $date1 or $delta are invalid, the
new date object will have an error associated with it.
Both of the calls above perform the same function and produce exactly the same
results.
If $subtract is not passed in, or is 0, the resulting date is formed as:
DATE2 = DATE1 + DELTA
If $subtract is non-zero, the resulting date is:
DATE2 = DATE1 - DELTA
The $subtract argument has special importance when doing approximate calculations, and
this is described below in "SUBTRACTION".
Delta/Delta calculations
Delta/delta calculations can be performed to add two amounts of time together, or
subtract them.
$delta3 = $delta1->calc($delta2 [,$subtract] [,$no_normalize]);
If $subtract is not passed in, or is 0, the resulting delta formed is:
DELTA3 = DELTA1 + DELTA2
If $subtract is non-zero, then the resulting delta is:
DELTA3 = DELTA1 - DELTA2
$delta1 and $delta2 are valid Date::Manip::Delta objects, and a new Date::Manip::Delta
object is produced.
$no_normalize can be the string 'nonormalize' or a non-zero value (in which case
$subtract MUST be entered, even if it is 0).
MODE
Date::Manip calculations can be divided into two different categories: business and non-
business; and within those are three sub-categories: exact, semi-exact, and approximate.
Business and non-business calculations
A business calculation is one where the length of the day is determined by the length
of the work day, and only business days (i.e. days in which business is conducted)
count. Holidays and weekends are omitted (though there is some flexibility in defining
what exactly constitutes the work week as described in the Date::Manip::Config
manual). This is described in more detail below in "BUSINESS MODE CONSIDERATIONS".
A non-business mode calculation is the normal type of calculation where no days are
ignored, and all days are full length.
Exact, semi-exact, and approximate calculations
An exact calculation is one in which the delta used (or produced) is an exact delta.
An exact delta is described in more detail in the Date::Manip::Delta manual, but the
short explanation is that it is a delta which only involves fields of an exactly known
length (hours, minutes, and seconds). Business deltas also include days in the exact
part. The value of all other fields in the delta will be zero.
A semi-exact calculation is one in which the deltas used (or produced) is a semi-exact
delta. This is also described in the Date::Manip::Delta manual, but the short
explanation is that it includes days and weeks (for standard calculations) or weeks
(for business calculations) in addition to the exact fields. A semi-exact day is
defined as the same clock time on two successive days. So noon to noon is 1 day (even
though it may not be exactly 24 hours due to a daylight saving time transition). A
week is defined as 7 days. This is described in more detail below.
An approximate calculation is one in which the deltas used (or produced) are
approximate, and may include any of the fields.
In date-delta and delta-delta calculations, the mode of the calculation will be determined
automatically by the delta. In the case of date-date calculations, the mode is supplied as
an argument.
Mode in date-date calculations
When doing a date-date calculation, the following call is used:
$delta = $date1->calc($date2 [,$subtract] [,$mode]);
$mode defaults to "exact". The delta produced will be be either a business or non-
business delta; exact, semi-exact, or approximate, as specified by $mode.
Currently, the possible values that $mode can have are:
exact : an exact, non-business calculation
semi : a semi-exact, non-business calculation
approx : an approximate, non-business calculation
business : an exact, business calculation
bsemi : a semi-exact, business calculation
bapprox : an approximate, business calculation
Mode in date-delta calculations
When doing calculations of a date and a delta:
$date2 = $date1->calc($delta [,$subtract]);
$date2 = $delta->calc($date1 [,$subtract]);
the mode is not passed in. It is determined exclusively by the delta. If $delta is a
business delta, A business calculation is done. If $delta is a non-business delta, a
non-business calculation will be done.
The $delta will also be classified as exact, semi-exact, or approximate based on which
fields are non-zero.
Mode in delta-delta calculations
When doing calculations with two deltas:
$delta3 = $delta1->calc($delta2 [,$subtract]);
the mode is not passed in. It is determined by the two deltas.
If both deltas are business mode, or both are non-business mode, a new delta will be
produced of the same type.
It one of the deltas is a business mode and the other is not, the resulting delta will
have an error condition since there is no direct correlation between the two types of
deltas. Even though it would be easy to add the two together, it would be impossible
to come up with a result that is meaningful.
If both deltas are exact, semi-exact, or approximate, the resulting delta is the same.
If one delta is approximate and one is not, then the resulting delta is approximate.
It is NOT treated as an error. Likewise, if one is semi-exact and the other exact, a
semi-exact delta is produced.
TIMEZONE CONSIDERATIONS
date-date calculations
When doing a business calculation, both dates must be in the same time zone or an
error is produced.
For non-business calculations, when calculating the difference between two dates in
different time zones, $date2 will be converted to the same timezone as $date1 and the
returned date will be in this timezone.
date-delta calculations
When adding a delta to a date, the resulting date will be in the same time zone as the
original date.
delta-delta calculations
No timezone information applies.
It should also be noted that daylight saving time considerations are currently ignored
when doing business calculations. In common usage, daylight saving time changes occurs
outside of the business day, so the business day length is constant. As a result,
daylight saving time is ignored.
BUSINESS MODE CONSIDERATIONS
In order to correctly do business mode calculations, a config file should exist which
contains the section defining holidays (otherwise, weekends will be ignored, but all other
days will be counted as business days). This is documented below, and in the
Date::Manip::Config section of the documentation. Some config variables (namely
WorkWeekBeg, WorkWeekEnd, WorkDayBeg, WorkDayEnd, and WorkDay24Hr) defined the length of
the work week and work day.
If the workday is defined as 08:00 to 18:00, a work week consisting of Mon-Sat, and the
standard (American) holidays, then from Tuesday at 12:00 to the following Monday at 14:00
is 5 days and 2 hours. If the "end" of the day is reached in a calculation, it
automatically switches to the next day. So, Tuesday at 12:00 plus 6 hours is Wednesday at
08:00 (provided Wed is not a holiday). Also, a date that is not during a workday
automatically becomes the start of the next workday. So, Sunday 12:00 and Monday at 03:00
both automatically becomes Monday at 08:00 (provided Monday is not a holiday).
Note that a business week is treated the same as an exact week (i.e. from Tuesday to
Tuesday, regardless of holidays). Because this means that the relationship between days
and weeks is NOT unambiguous, when a semi-exact delta is produced from two dates, it will
be in terms of d/h/mn/s (i.e. no week field).
Anyone using business mode is going to notice a few quirks about it which should be
explained. When I designed business mode, I had in mind what a business which promises 1
business day turnaround really means.
If you do a business calculation (with the workday set to 9:00-17:00), you will get the
following:
Saturday at noon + 1 business day = Tuesday at 9:00
Saturday at noon - 1 business day = Friday at 9:00
What does this mean?
As an example, say I use a business that works 9-5 and they have a drop box so I can drop
things off over the weekend and they promise 1 business day turnaround. If I drop
something off Friday night, Saturday, or Sunday, it doesn't matter. They're going to get
started on it Monday morning. It'll be 1 business day to finish the job, so the earliest
I can expect it to be done is around 17:00 Monday or 9:00 Tuesday morning. Unfortunately,
there is some ambiguity as to what day 17:00 really falls on, similar to the ambiguity
that occurs when you ask what day midnight falls on. Although it's not the only answer,
Date::Manip treats midnight as the beginning of a day rather than the end of one. In the
same way, 17:00 is equivalent to 9:00 the next day and any time the date calculations
encounter 17:00, it automatically switch to 9:00 the next day. Although this introduces
some quirks, I think this is justified. I also think that it is the way most people think
of it. If I drop something off first thing Monday morning, I would expect to pick it up
first thing Tuesday if there is 1 business day turnaround.
Equivalently, if I want a job to be finished on Saturday (despite the fact that I cannot
pick it up since the business is closed), I have to drop it off no later than Friday at
9:00. That gives them a full business day to finish it off. Of course, I could just as
easily drop it off at 17:00 Thursday, or any time between then and 9:00 Friday. Again,
it's a matter of treating 17:00 as ambiguous.
So Saturday + 1 business day = Tuesday at 9:00 (which means anything from Monday 17:00 to
Tuesday 9:00), but Monday at 9:01 + 1 business day = Tuesday at 9:01 which is unambiguous.
It should be noted that when adding years, months, and weeks, the business day is ignored.
Once they've been added, the resulting date is forced to be a business time (i.e. it moves
to the start of the next business day if it wasn't one already) before proceeding with the
days, hours, minutes, and seconds part.
EXACT, SEMI-EXACT, AND APPROXIMATE DATE/DELTA CALCULATIONS
This section contains more details about exactly how exact, semi-exact, and approximate
calculations are performed for date/delta calculations.
All calculations make use of some exact quantities, including:
1 year = 12 months
1 week = 7 days
1 hour = 60 minutes
1 minute = 60 seconds
This leaves two relationships which are not exact:
1 month = ? days
1 day = ? hours
For non-business calculations, a day is usually 24 hours long. Due to daylight saving time
transitions which might make a day be 23 or 25 hours long (or in some cases, some other
length), the relation is not exact. Whenever possible, a day is actually measured as the
same time on two days (i.e. Tuesday at noon to Wednesday at noon) even if that period is
not precisely 24 hours. For business calculations, a days length is determined by the
length of the work day and is known exactly.
Exact calculations involve ONLY quantities of time with a known length, so there is no
ambiguity in them.
Approximate and semi-exact calculations involve variable length fields, and so they must
be treated specially.
In order to do an approximate or semi-exact calculation, the delta is added to a date in
pieces, where the fields in each piece have an exact and known relationship.
For a non-business calculation, a calculation occurs in the following steps:
year/month fields added
week/day fields added
hour/minute/second fields added
For a business calculation, the steps are:
year/month fields added
week field added
day field added
hour/minute/second fields added
After each step, a valid date must be present, or it will be adjusted before proceeding to
the next step. Note however that for business calculations, the first step must produce a
valid date, but not necessarily a business date. The second step will produce a valid
business date.
A series of examples will illustrate this.
A date and non-business approximate delta
date = Mar 31 2001 at 12:00:00
delta = 1 year, 1 month, 1 day, 1 hour
First, the year/month fields are added without modifying any other field. This would
produce:
Apr 31, 2002 at 12:00
which is not valid. Any time the year/month fields produce a day past the end of the
month, the result is 'truncated' to the last day of the month, so this produces:
Apr 30, 2002 at 12:00
Next the week/day fields are added producing:
May 1, 2002 at 12:00
and finally, the exact fields (hour/minute/second) are added to produce:
May 1, 2002 at 13:00
A simple business calculation
Assuming a normal Monday-Friday work week from 8:00 - 17:00:
date = Wed, Nov 23, 2011 at 12:00
delta = 1 week, 1 day, 1 hour
First, the week field is added:
Wed, Nov 30, 2011 at 12:00
Then the day field is added:
Thu, Dec 1, 2011 at 12:00
Then the exact fields are added:
Thu, Dec 1, 2011 at 13:00
A business example where a holiday impacts it
In America, Jul 4 is a holiday, so Mon, Jul 4, 2011 is not a work day.
date = Mon, Jun 27, 2011 at 12:00
delta = 1 week, 1 day, 1 hour
First, the week field is added:
Mon, Jul 4, 2011 at 12:00
Since that is not a work day, it immediately becomes:
Tue, Jul 5, 2011 at 8:00
Then the day field is added:
Wed, Jul 6, 2011 at 8:00
and finally the remaining fields:
Wed, Jul 6, 2011 at 9:00
Calculation where daylight savings time impacts it (fall example)
In the America/New_York timezone (Eastern time), on November 6, 2011, the following
time change occurred:
2011-11-06 02:00 EDT => 2011-11-06 01:00 EST
Three simple calculations illustrate how this is handled:
date = 2011-11-05 02:30 EDT
delta = 1 day
Adding the day produces:
2011-11-06 02:30 EDT
which is valid, so that is the result.
Similarly:
date = 2011-11-07 02:30 EST
delta = -1 day
produces:
2011-11-06 02:30 EST
which is valid.
Finally:
date = 2011-11-05 02:30 EDT
delta = 2 days
produces:
2011-11-07 02:30 EST
The calculation will preserve the savings time where possible so the resulting day
will have the same offset from UTC. If that is not possible, but the resulting day is
valid in the other offset, that will be used instead.
Calculation where daylight savings time impacts it (spring example)
In the America/New_York timezone (Eastern time), on March 13, the following time
change occurred:
2011-03-13 02:00 EST => 2011-03-13 03:00 EDT
In this case, a calculation may produce an invalid date.
date = 2011-03-12 02:30 EST
delta = 1 day
produces:
2011-03-13 02:30 EST
This is not valid. Neither is:
2011-03-13 02:30 EDT
In this case, the calculation will be redone converting days to 24-hour periods, so
the calculation becomes:
date = 2011-03-12 02:30 EST
delta = 24 hours
which will produce a valid date:
2011-03-13 03:30 EDT
EXACT, SEMI-EXACT, AND APPROXIMATE DATE/DATE CALCULATIONS
This section contains more details about exactly how exact, semi-exact, and approximate
calculations are performed for date/date calculations.
When calculating the delta between two dates, the delta may take different forms depending
on the mode passed in. An exact calculation will produce a delta which included only exact
fields. A semi-exact calculation may produce a semi-exact delta, and an approximate
calculation may produce an approximate delta. Note that if the two dates are close enough
together, an exact delta will be produced (even if the mode is semi-exact or approximate),
or it may produce a semi-exact delta in approximate mode.
For example, the two dates "Mar 12 1995 12:00" and "Apr 13 1995 12:00" would have an exact
delta of "744 hours", and a semi-exact delta of "31 days". It would have an approximate
delta of "1 month 1 day".
Two dates, "Mar 31 12:00" and "Apr 30 12:00" would have deltas "720 hours" (exact), "30
days" (semi-exact) or "1 month" (approximate).
Approximate mode is a more human way of looking at things (you'd say 1 month and 2 days
more often then 33 days), but it is less meaningful in terms of absolute time.
One thing to remember is that an exact delta is exactly the amount of time that has
passed, including all effects of daylight saving time. Semi-exact and approximate deltas
usually ignore the affects of daylight saving time.
SUBTRACTION
In exact and semi-exact calculations, and in delta-delta calculations, the the $subtract
argument is easy to understand. When working with an approximate delta however (either
when adding an approximate delta to a date, or when taking two dates to get an approximate
delta), there is a degree of uncertainty in how the calculation is done, and the $subtract
argument is used to specify exactly how the approximate delta is to be use. An example
illustrates this quite well.
If you take the date Jan 4, 2000 and subtract a delta of "1 month 1 week" from it, you end
up with Nov 27, 1999 (Jan 4, 2000 minus 1 month is Dec 4, 1999; minus 1 week is Nov 27,
1999). But Nov 27, 1999 plus a delta of "1 month 1 week" is Jan 3, 2000 (Nov 27, 1999 plus
1 month is Dec 27, 1999; plus 1 week is Jan 3, 2000).
In other words the approximate delta (but NOT the exact or semi-exact delta) is different
depending on whether you move from earlier date to the later date, or vice versa. And
depending on what you are calculating, both are useful.
In order to resolve this, the $subtract argument can take on the values 0, 1, or 2, and
have different meanings.
$subtract in approximate date-date calculations
In the call:
$delta = $date1->calc($date2,$subtract,"approx");
if $subtract is 0, the resulting delta can be added to $date1 to get $date2. Obviously
$delta may still be negative (if $date2 comes before $date1).
If $subtract is 1, the resulting delta can be subtracted from $date1 to get $date2
(the deltas from these two are identical except for having an opposite sign).
If $subtract is 2, the resulting delta can be added to $date2 to get $date1. In other
words, the following are identical:
$delta = $date1->calc($date2,2,"approx");
$delta = $date2->calc($date1,"approx");
$subtract in approximate date-delta calculations
In the call:
$date2 = $date1->calc($delta,$subtract);
If $subtract is 0, the resulting date is determined by adding $delta to $date1.
If $subtract is 1, the resulting date is determined by subtracting $delta from $date1.
If $subtract is 2, the resulting date is the date which $delta can be added to to get
$date1.
For business mode calculations, $date1 will first be adjusted to be a valid work day
(if it isn't already), so this may lead to non-intuitive results.
In some cases, it is impossible to do a calculation with $subtract = 2. As an
example, if the date is "Dec 31" and the delta is "1 month", there is no date which
you can add "1 month" to to get "Dec 31". When this occurs, the date returned has an
error flag.
APPROXIMATE DATE/DATE CALCULATION
There are two different ways to look at the approximate delta between two dates.
In Date::Manip 5.xx, the approximate delta between the two dates:
Jan 10 1996 noon
Jan 7 1998 noon
was 1:11:4:0:0:0:0 (or 1 year, 11 months, 4 weeks). In calculating this, the first date
was adjusted as far as it could go towards the second date without going past it with each
unit starting with the years and ending with the seconds.
This gave a strictly positive or negative delta, but it isn't actually how most people
would think of the delta.
As of Date::Manip 6.0, the delta is 2:0:0:-3:0:0:0 (or 2 years minus 3 days). Although
this leads to mixed-sign deltas, it is actually how more people would think about the
delta. It has the additional advantage of being easier to calculate.
For non-business mode calculations, the year/month part of the approximate delta will move
a date from the year/month of the first date into the year/month of the second date. The
remainder of the delta will adjust the days/hours/minutes/seconds as appropriate.
For approximate business mode calculations, the year, date, and week parts will be done
approximately, and the remainder will be done exactly.
KNOWN BUGS
None known.
BUGS AND QUESTIONS
Please refer to the Date::Manip::Problems documentation for information on submitting bug
reports or questions to the author.
SEE ALSO
Date::Manip - main module documentation
LICENSE
This script is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.
AUTHOR
Sullivan Beck (sbeck@cpan.org)