Little know fact, is that if you disable time synchronisation on the Guest with VM Tools, there are still times when that time sync will occur!
I found the issue when the AVVI backup agent for Backup Exec, tried to snapshot a VM that had been recently moved on to a new host. that Host did not have its time and time zone configured correctly, so the backup failed as the time was sent back 20 hours. Once the backup failed the windows time service synced the machine back to the correct time
Timekeeping best practices for Windows, including NTP (1318)
Disabling Time Synchronization (1189)