From Date::Manip docs:

Date::Manip must be able to determine the timezone the user is in. It does this by looking in the following places:

   $Date::Manip::TZ (set with Date_Init or in Manip.pm)
   $ENV{TZ}
   the unix `date` command (if available)
   $main::TZ
   /etc/TIMEZONE
   /etc/timezone
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Before running your script,
In Windows, do set TZ=...timezone...
In unix, do export TZ=...timezone...