I don't know if it's a rom issue, but I doubt it. I'm running AMDZero Base v1.2 (or whatever the latest is) with DG21.
I've had this problem with other phones, but I was able to disable the auto update from the carrier... but that doesn't seem to work here.
- Changing the timezone in the Windows Mobile clock setting
- Changing the timezone in the Omnia settings dialog
- Changing the timezone in the Omnia clock program and unselecting the auto update