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3.7. Parsing Dates and Times from Strings

3.7.1. Problem

You read in a date or time
specification in an arbitrary format but need to parse that string
into distinct year, month, etc. values.

3.7.2. Solution

If your date is already numeric, or in a rigid and easily parsed
format, use a regular expression (and possibly a hash mapping month
names to numbers) to extract individual day, month, and year values,
and then use the standard Time::Local module''s
timelocal and timegm functions
to turn that into an Epoch seconds value.

use Time::Local;
# $date is "2003-02-13" (YYYY-MM-DD form).
($yyyy, $mm, $dd) = ($date =~ /(\d+)-(\d+)-(\d+)/);
# calculate epoch seconds at midnight on that day in this timezone
$epoch_seconds = timelocal(0, 0, 0, $dd, $mm-1, $yyyy);

For a
more flexible solution, use the ParseDate function
provided by the CPAN module Date::Manip, and then use
UnixDate to extract the individual values.

use Date::Manip qw(ParseDate UnixDate);
$date = ParseDate($STRING);
if (!$date) {
# bad date
} else {
@VALUES = UnixDate($date, @FORMATS);
}

3.7.3. Discussion

The flexible ParseDate function accepts many
formats. It even converts strings such as "today", "2 weeks ago
Friday", "2nd Sunday in 1996", and "last Sunday in December", plus it
understands the date and time format used in mail and news headers.
It returns the decoded date in its own format: a string of the form
"YYYYMMDDHH:MM:SS". You could compare two such strings to compare the
dates they represent, but arithmetic is difficult. We therefore use
the UnixDate function to extract the year, month,
and day values in a preferred format.

UnixDate
takes a date (as returned by ParseDate) and a list
of formats. It applies each format to the string and returns the
result. A format is a string describing one or more elements of the
date and time and the way that the elements are to be formatted. For
example, %Y is the format for the year in
four-digit form. Here''s an example:

use Date::Manip qw(ParseDate UnixDate);
while (<>) {
$date = ParseDate($_);
if (!$date) {
warn "Bad date string: $_\n";
next;
} else {
($year, $month, $day) = UnixDate($date, "%Y", "%m", "%d");
print "Date was $month/$day/$year\n";
}
}

3.7.4. See Also

The documentation for the CPAN module Date::Manip; we use this in
Recipe 3.11