The workspace calendar shows events, tasks, and milestones based on the time zone setting of the member who is viewing it. For example, if a member who is in the Eastern Daylight Time Zone (GMT-5:00) creates an event that starts at 12:00PM, a member who is in the Pacific Daylight Time Zone (GMT-8:00) will see the event in the calendar as starting at 9:00AM.
Each member's time zone setting is specified in the account profile. To change it, go to Account > My Account > Preferences.
Be sure to click the Update Profile button at the bottom to save any changes.