Well the obvious question is, has it always worked okay in the past? And what is the time shown on the phones?
Meanwhile...
"Does the VMP server on our PC update its own time seperate from the IPO?" I assume that mean that you have Voicemail Pro running on a Windows PC. In that case, the voicemail is taking its time from the PC, so check the time on that PC.
The IP Office may be taking its time from a number of different possible sources - but in that scenario, its probably best to have it take the time from the voicemail service, so that they are always in-sync (whether in-sync wrong or in-sync correct but at least in-sync and the time on phone displays will tell you the time on both). That's done through the IP Office system configuration, setting the Time Setting Config Source to Voicemail Pro/Manager.
It could be possible that what you have on the PC is just the Voicemail Pro client and your voicemail is on a UCM module inside the IP Office system. In that case, unless someone has messed with the UCM configuration, the UCM gets its time from the IP Office, so check the IP Office settings.
Stuck in a never ending cycle of file copying.