Hershel,
I have never done this myself, but if the MBG is capable of changing the time than most likely just giving the phone a different NTP Server may do it. When you setup the phones DHCP Options, Option 42, to give it a different NTP Server. This may require that you shut down the NTP Service within the 5000 and do this for all your sites.
Alternately you might be able to use the HTML Software to modify that as well, but then again I have never tried that either.
Thanks,
TE