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```Date::Manip::Calc(3)  User Contributed Perl Documentation Date::Manip::Calc(3)

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:

Perceived: 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 wall clock	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	occurring.
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:
exact, semi-exact, and approximate.

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

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
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
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

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
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.

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
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
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:

For a business calculation, the steps are:

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

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

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

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,

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.

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
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.

Date::Manip	  - main module	documentation