If the guest VM should always boot with the same date/time on the RTC, two actions must be done:
1. Edit the .vmx file of the guest VM, and add a line like this:
rtc.starttime = "1238765436"
where the number is a 1970-based epoch, in seconds.
2. Boot the guest VM, and run VMtools. Disable the host-to-guest time synchronization, and reboot the guest VM.
Then, the guest VM will always boot with the same date and time in the calendar.