Garmin GPS IIplus Owner's Manual page 80

Reference
Fig. 74a
Fig. 74b
74
Date and Time Setup
The date and time is located directly below the mode field. Note: Date and time
information is derived from the GPS satellites and cannot be changed by the user.
Because the time shown is UTC (Greenwich mean time) time, you will need to enter
a time offset to display the correct local time for your area. To determine the time
offset for your area, note your position and refer to the chart in Appendix F.
To enter the time offset:
1. Highlight the 'offset' field (Fig. 74a), and press ENTER.
2. Enter the time offset, and press ENTER.
Note: Move the cursor to the left to change the plus or minus sign.
Time Format
The time display shown on the system setup and Position Pages may be set to
display the time in a 12- or 24-hour format.
To select the time format:
1. Highlight the 'hours' field (Fig. 74b), and press ENTER.
2. Select 12- or 24-hour display, and press ENTER.