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25.8. Dates and Times

Where possible, use the ISO-8601 standard ("YYYY-MM-DD") to represent
dates. Strings like "1987-12-18" can be easily converted into a
system-specific value with a module like Date::Parse. A list of
time and date values (such as that returned by the built-in localtime
function) can be converted to a system-specific representation using
Time::Local.

The built-in time function will always return the number of seconds
since the beginning of the "epoch", but operating systems differ in
their opinions of when that was. On many systems, the epoch began on
January 1, 1970, at 00:00:00 UTC, but it began 66 years earlier on Macs,
and on VMS it began on November 17, 1858, at 00:00:00. So for portable
times you may want to calculate an offset for the epoch:

require Time::Local;
$offset = Time::Local::timegm(0, 0, 0, 1, 0, 70);

A system''s representation of the time of day and the calendar date can
be controlled in widely different ways. Don''t assume the time zone is
stored in $ENV{TZ}. Even if it is, don''t assume
that you can control the time zone through that variable.