The FAA doesn't recognize "hobbs" time or "tach" time as an official method of time keeping for any purpose. Those are considered estimates of flight time and time in service which are official FAA terms for recording pilot time and aircraft time as defined in Part 1 of regs.

If forced to make a ruling the most correct legal answer to your question would be "time in service" but either "hobbs" or "tach" would be an acceptable estimate of that time.