If you're talking full CallManager, then change the time on your publisher (your CallManagers subscibers should sync to it, and your phones should sync to the CallManager they register to).
If your CallManager services multiple time zones it's a little more involved (but not much).
If you have CME, then you set the time on the router - but it's lost when you reboot. So you're best to set an NTP server:
ntp server x.x.x.x
(if you don't have an NTP box then google for an internet one).
You can also use NTP with CallManager - see this:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_configuration_example09186a008009470f.shtml
Regards
Aaron
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Aaron
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