UnZip requires the host computer to have proper timezone information in
order to handle certain tasks correctly (see unzip.doc).  To set the
timezone on the Macintosh, go to the Map Control Panel and enter the
correct number of hours (and, in a few locales, minutes) offset from
Universal Time/Greenwich Mean Time.  For example, the US Pacific timezone
is -8 hours from UTC/GMT during standard (winter) time and -7 hours from
UTC/GMT during Daylight Savings Time.  The US Eastern timezone is -5 hours
during the winter and -4 hours during the summer.

(Corrections welcome...description is loosely based on information from
Johnny Lee.)