Similar problem came up recently with a bbs running on a server in the U.S. for a Japanese audience. Daylight savings time and so on were involved. As long as you stick to unix time (seconds from epoch) you can calculate things like "24 hours ago" easily. Staying in Greenwich time until the end is useful.

But if you want to do lots of other calculations (like I did for a recent timesheet tracking program that prints a calendar on top of a form) Date::Manip can do just about anything. Careful, it and CGI are some of the biggest modules in Perldom. (My process was 9 mb).