It's called epoch time, the number of seconds since Jan 1st, 1970.

>perl -le "print scalar gmtime 1138597506
Mon Jan 30 05:05:06 2006
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To convert to epoch time, you can use core module Time::Local's timegm or timelocal.

To make changes, you should use Date::Calc.

To render into a human-readable format, POSIX's strftime (in conjunction with localtime or gmtime) should do the trick.

Time::ParseDate might be of use if you want to avoid the dropdowns.

Update: Rearranged order of text. Added last paragraph.