This may also work on other Cisco models as well, but may be found in a different location.
On the Cisco phone web GUI, under the Regional Tab, scroll to the bottom and in the Time section, place the following text in the Daylight Saving Time Rule box.
start=3/8/7/02:0:0;end=11/1/7/02:0:0;save=1
If you want 24hr display instead of 12hr, change that on the User tab on the right.