More info. Have gone as far as editing sip.cfg file to show daylight savings enable = 0. Also factory-defaulted both Polycom and Adtran 706 phones. Adtran shows correct time; Polycom does not.
It seems I found a solution from viewing an online posting from an Asterisk handbook. That info referred to editing sections of locally-unique configuration files. From that, and given the file syntax was quite similar,I intuited the following syntax to enter in the 7100's GUI under IP Phone Globals\Global Files\Polycom Customer-sip.cfg tab. You should be able to discern what was added versus what is default:
<?xml version="1.0" standalone="yes"?>
<!-- Customer SIP Application Configuration File -->
After factory-defaulting the Polycom phone, it displayed the correct time for Arizona.