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

At least one of these should contain a timezone 
in one of the supported forms.  
If none do by default, the TZ variable must be set with Date_Init.