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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 date calculations. Date 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 date calculations:

       Date-Date calculations
	      $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 is produced and is described below.

       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.

       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).

MODE
       Date::Manip calculations can be divide 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.

	   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 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 alculation
	      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 the exact, semi-exact, and approx 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
       In many cases, it is somewhat ambiguous what amount of time a delta actually refers to.
       Some relationships between fields in the delta are known.  These include:

	 1 year   = 12 months
	 1 week   = 7 days
	 1 hour   = 60 minutes
	 1 minute = 60 seconds

       Other relationships are not known.  These include:

	 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
       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 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 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 the meanings described next.

       $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)

perl v5.16.3				    2014-06-09			     Date::Manip::Calc(3)
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