Recipe 3.4 Adding to or Subtracting from a Date
3.4.1 Problem
You
have a date and time and want to find the date and time of some
period in the future or past.
3.4.2 Solution
Simply add or subtract Epoch seconds:
$when = $now + $difference;
$then = $now - $difference;
If
you have distinct DMYHMS values, use the CPAN Date::Calc module. If
you're doing arithmetic with days only, use
Add_Delta_Days ($offset is a
positive or negative integral number of days):
use Date::Calc qw(Add_Delta_Days);
($y2, $m2, $d2) = Add_Delta_Days($y, $m, $d, $offset);
If you are concerned with hours, minutes, and seconds (in other
words, times as well as dates), use
Add_Delta_DHMS:
use Date::Calc qw(Add_Delta_DHMS);
($year2, $month2, $day2, $h2, $m2, $s2) =
Add_Delta_DHMS( $year, $month, $day, $hour, $minute, $second,
$days_offset, $hour_offset, $minute_offset, $second_offset );
3.4.3 Discussion
Calculating with Epoch seconds is easiest, disregarding the effort to
get dates and times into and out of Epoch seconds. This code shows
how to calculate an offset (55 days, 2 hours, 17 minutes, and 5
seconds, in this case) from a given base date and time:
$birthtime = 96176750; # 18/Jan/1973, 3:45:50 am
$interval = 5 + # 5 seconds
17 * 60 + # 17 minutes
2 * 60 * 60 + # 2 hours
55 * 60 * 60 * 24; # and 55 days
$then = $birthtime + $interval;
print "Then is ", scalar(localtime($then)), "\n";
Then is Wed Mar 14 06:02:55 1973
We could have used Date::Calc's Add_Delta_DHMS
function and avoided the conversion to and from Epoch seconds:
use Date::Calc qw(Add_Delta_DHMS);
($year, $month, $day, $hh, $mm, $ss) = Add_Delta_DHMS(
1973, 1, 18, 3, 45, 50, # 18/Jan/1973, 3:45:50 am
55, 2, 17, 5); # 55 days, 2 hrs, 17 min, 5 sec
print "To be precise: $hh:$mm:$ss, $month/$day/$year\n";
To be precise: 6:2:55, 3/14/1973
As usual, we need to know the range of values the function expects.
Add_Delta_DHMS takes a full year value—that
is, one that hasn't had 1900 subtracted from it. The month value for
January is 1, not 0. Date::Calc's Add_Delta_Days
function expects the same kind of values:
use Date::Calc qw(Add_Delta_Days);
($year, $month, $day) = Add_Delta_Days(1973, 1, 18, 55);
print "Nat was 55 days old on: $month/$day/$year\n";
Nat was 55 days old on: 3/14/1973
3.4.4 See Also
The documentation for the CPAN module Date::Calc
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