Try it again with  timegm() - you might be getting a daylight savings transition.

From the Time::Local docs:

       We just
       assume that we're translating a GMT time, and then fudge
       it when we're done for the timezone and daylight savings
       arguments.  Note that the timezone is evaluated for each
       date because countries occasionally change their official
       timezones.  Assuming that localtime() corrects for these
       changes, this routine will also be correct.  The daylight
       savings offset is currently assumed to be one hour.
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