Once you have a position fix and a good lock on the GPS signals, the receiver should be using that info to give you the correct time. You’ll just need to make time zone and daylight saving time settings, but due to the recent changes to daylight saving time in the US, you may need to experiment a little.
From the manual:
Changing the Time Zone
To change the time zone:
1. Press the PAGE button to switch to the MENU Page. Press the UP or DOWN button to highlight ‘SETUP,’ then press ENTER. The SETUP Page appears.
2. Press the UP or DOWN button to highlight ‘TIME,’ then press ENTER.
3. Press the UP or DOWN button to select the ‘TIME ZONE’ field, then press ENTER.
4. Press the UP or DOWN button to scroll through the selections. Press ENTER when you have found your correct time zone. Press PAGE to get back to the page of your choice.
You can also change the time format and UTC (Greenwich time) offset by choosing “Time” from the Setup menu. For more info, check out the manual.