When he goes to school as an AD officer the minute he arrives that counts to his commitment time. If the school has a commitment period associated with it that is longer than the NROTC commitment it will run concurrent and it will start the day he GRADUATES FROM NUKE SCHOOL. If he does not graduate it will revert back to the original commitment.
Example:
UPT is 10 yrs. You step foot on the base 6 months after commissioning (let's say Nov 1st).
UPT is 1 yr long. You bust in July, and do not graduate, reassigned to a field that does not have additional commitment period. Your release date would revert to Nov., not the May date(commissioning).
or
You graduate Nov. one yr later. (18 months after commissioning), your commitment clock of 10 yrs starts on that day. In essence, you will serve 11 1/2 yrs, in total even though your NROTC commitment would have ended yrs earlier.
Hope that helps.