Many people give Murtha credit for 39 years in the military, but he retired in 1990 with 27 years combined = 4 yrs Active Duty/23 yrs Reserve (Weekend Warrior) time,
Not sure where you got your education nor what type math they taught but he has more than 27 yrs by the data your posted.
Enlisted in 1952, retired in 1990 with continious service either in the active corps or the reserve.
1990 - 1952 = more than 27.