GMC 2010 Yukon/Yukon XL Owner's Manual page 304

5. To decrease the time or date, do one of the
following:
Press the SEEK button.
Press the REV button.
Turn the knob counterclockwise.
To change the time default setting from 12 hour to
24 hour or to change the date default setting from
month/day/year to day/month/year:
1. Press the button and then the softkey located
below the forward arrow tab. 12H, 24H, the date
MM/DD (month and day), and DD/MM/ (day and
month) displays.
2. Press the softkey located below the desired option.
3. Press the button again to apply the desired
option, or let the screen time out.
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