Your SignUp assignment's calendar sync will post in the same time zone as where the Organizer resides. If you're seeing a discrepancy, please see this article published by Google:
https://support.google.com/calendar/answer/37834?hl=en
"Note that some calendar applications don't export their calendars with time zone information. If this is the case and your Google Calendar is in a different time zone than the calendar you're importing, this may result in your events not showing up in your current time zone."