Although there is no way to change the voicemail timestamps, there is a way to change the time displayed on each Polycom phone.
Polycom phones allow the Time to be customized by accessing the built in web interface within the Polycom phone.
Browse to the IP address of the Polycom phone and Login to the Polycom Web Interface using username: Polycom password: 456
Click the General tab then Click on Time in the subMenu
Insert the hostname or IP of the NTP server you wish to sync with in the SNTP Server field, if unsure leave this set to http://pool.ntp.org
Set the GMT offset for the timezone this phone is in (Example: Central Timezone is -6, Pacific is -8)
Click Submit, The Polycom Phone should reboot. If it does not, reboot manually.
Once the phone is back up, the time will be displayed for the selected timezone.