When does service time start in the U.S. Navy on entry to the Naval Academy or upon graduation?

Your service time begins when you report for active duty - in the case of the Naval Academy, it's when you report for duty as a Plebe.

The Academy is just an "extended" boot camp and college for officers in training. Recruits with degrees would normally attend OCS, and go through a shorter program to just learn how to be an officer. For enlisted, boot camp is it since there is no degree involved, or at least most of the time. There are those with degrees who choose not to get a commission.

I hate to disagree with you about that but unless the Navy has changed, our time at the Academy did NOT count towards your pay and benefits nor towards your retirement. As neither an enlisted nor an officer you were in between official ranks. Now I was in the Class of 1979 and as a prior enlisted I was a little bit down to find that it didn't do anything towards our longevity. Yes, we were active duty, yes we did have a green id card instead of a red one but that time did not count. Hoss55