Can you explain the purpose of your report? Are these times being used in Workfront projects, such as in Planned Completion Dates? Do you want to see all the times people will be completing tasks in various timezones? If that's what you're looking for it may help to know that Workfront will show time values based on your computer's timezone setting. So if you're in New York you may see a task due to be completed at 4:00 pm while a user in Los Angeles looking at the same task would see that it was due at 1:00 pm.
I hope that helps, but if you're trying to create a report where people enter times in different time zones, Workfront will not allow them to choose a time zone, it will just assume the time zone that the computer is set to.