$siteDefaults{timezone} = "America/Denver";
in the global.conf directory
DateTime uses a continent/city approach rather than time zones because
the actual times can vary within one time zone (Indiana is particularly
exciting. :-) )
Pointers to the DateTime list of specialized "timezones" is in the help balloon
shown next to the TimeZone section of the Course Configuration page (last link in "Main menu" of WeBWorK.)
See http://en.wikipedia.org/wiki/Zoneinfo and http://en.wikipedia.org/wiki/List_of_zoneinfo_time_zones