Is it legal for a A/C to not have some kind of clock to measure MR time.
Yes
is it possible for the pilot to "guesstimate" this time?
No.
A timepiece is a mandatory instrument - either installed on the panel or worn by the pilot. The pilot is required to keep a flight log. This time can be entered in the MR.
I along with everyone I know diligently looks at their watch to determine the flight time for the MR.
