Boxtops:
The time at the International Date Line is either 12hours ahead of, or 12 hours behind, GMT (depending upon whether you're to the West or the East of the line).
Mrs LB:
Are you simply changing the time on the clock or changing the time zone? Right-click on the clock and select 'Adjust Date/Time'. Click on the 'Time Zone' tab. Use the drop-down menu to select 'GMT'. Then use the 'Date & Time' tab to correct the clock. Then (IMPORTANTLY) click on 'APPLY' (NOT just on 'OK'. Then click 'OK'.
Chris