Re: date

[sierrathedog04]

Normally the solution would be simply:

my $yesterday = time() - (24 * 60 * 60);
my ($day, $month, $year) = localtime($yesterday)[3,4,5];

(Remember to add one to the value of $month and 1900 to the value of $year before printing them out.)

However, yesterday's date may not be twenty-four hours ago. The day on which daylight savings time (DST) ends is 25 hours long; the day on which DST begins is only 23 hours long.

Thus if you calculated the date 24 hours earlier during the last hour of the day DST ended you would get an erroneous result using the above formula, i.e., "yesterday's" and today's dates would be the same.

Similarly if you calculated the date 24 hours earlier during the first hour of the first whole day after DST began you would also get an erroneous result using the above formula: "yesterday's" date would be two days before today's date.

'perldoc perlfaq4' has a workaround by Russ Allbery that fixes the DST problem.

Update: Corrected above to state that the formula at top would provide incorrect results in the last hour of the day DST ended, not the first hour of the day after it ended.

[mattr]

Similar problem came up recently with a bbs running on a server in the U.S. for a Japanese audience. Daylight savings time and so on were involved. As long as you stick to unix time (seconds from epoch) you can calculate things like "24 hours ago" easily. Staying in Greenwich time until the end is useful.

But if you want to do lots of other calculations (like I did for a recent timesheet tracking program that prints a calendar on top of a form) Date::Manip can do just about anything. Careful, it and CGI are some of the biggest modules in Perldom. (My process was 9 mb).