Working with dates is a common enough task and Perl comes with a couple of built-in functions for lower level date calculations (localtime() and gmtime()) and a Time::Local module which provides complementary functions (timelocal() and timegm()). However, for doing higher level date calculations you’ll want to grab either the Date::Manip or Date::Calc modules from CPAN.

The Date::Manip module is pure Perl (so it’ll install and run anywhere you have Perl) and is quite large. The Date::Calc module uses a C library so you’ll need to compile your own or get a prebuilt version (activestate has a version for win32). Date::Calc is leaner, much faster, and probably the one you want to work with (we’ll use it in the examples here).

The Date::Calc module contains a large number of convenience routines such as: Day_of_Year(), check_date(), leap_year(), Delta_Days(), Date_to_Text(), and too many more to list. For example, to check to see if a given date is valid:

    use Date::Calc qw/check_date/;
    print "Bad Date\n" unless check_date(2001, 9, 31);

This tells us that the date is bad because there is no September 31st. We can calculate the difference between two dates with the Delta_Days function:

    use Date::Calc qw/Delta_Days/;
    my ($year1, $month1, $day1) = (2001, 2, 8);
    my ($year2, $month2, $day2) = (2001, 12, 25);
    my $delta = Delta_Days($year1, $month1, $day1,
                           $year2, $month2, $day2);
    print "Only $delta more days until Christmas!\n";

Have you been told your invoice will be paid in 45 days and want to circle that date on your calendar?

    use Date::Calc qw/Add_Delta_Days Date_to_Text_Long/;
    my ($year, $month, $day) = (localtime())[5,4,3];
    $year += 1900;
    $month++;
    print "Enter number of days from today: ";
    chomp(my $ddays = <STDIN>);
    my $date = Date_to_Text_Long(
                    Add_Delta_Days($year, $month, $day, $ddays)
               );
    print "$date is $ddays from today\n";

You can also get or add deltas with hours, minutes, and seconds for more precise calculations. And, of course, there is a good deal more functionality in the Date::Calc, and the Date::Manip modules than I’ve highlighted here.

Working with dates isn’t hard, but it also isn’t hard to work with them incorrectly — using one of the Date modules helps you avoid mistakes and often greatly simplifies your task.

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