Because calendar rules are influenced by historical and political decisions, date manipulations are riddled with pitfalls. Perl’s DateTime module knows all the tricks.
If a backup script launches at 10 pm and quits at 4 am, how long did it take to run? Six hours? Well, it depends. Just think about a process that ran between March 26 and 27 in 2005 somewhere in the UK. The clocks were put forward by one hour at 1 am, and that would make five hours the right answer. If the same process had run at the same time in the USA, the answer would have been six hours, as summer time starts a week later in the US. But not in Indiana, which had not yet introduced summer time in 2005. In fact, Indiana is introducing summer time this year (2006, [2]). Fortunately, the DateTime module ([5]) from CPAN knows all these historical and future rules and provides an easy interface to even the most complex date calculations. What if you wanted to know how long the current summer time rules have been in use in the UK? Listing 1 (dsthist) discovers this by looking back from the year 2006 and checking through the month of March to find out if there is a day where you end up at 5 a.m. when adding three hours and one second to 00:59:59. If this happens, summer time was used during this year, and the script stops when it discovers that’s not the case. The display shows that 1972 was the first year with today’s summertime rules:
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