Workflow to Add/Update Field Value calculated from another Field Value

Hello, I am trying to update a Record’s Field’s Value that is derived from another Field Value. The source Field Value is a Date & Time, and the target Field Value is the same Date & Time except it is dialed back 3 hours earlier than the source value. So far I have been unsuccessful at updating a Field Value using a workflow triggered after the creation of a Record. Is there any example workflow I can reference for this task?

Thanks!