WordNet (r) 3.0 (2006):

tour of duty
    n 1: a period of time spent in military service [syn:
         enlistment, hitch, term of enlistment, tour of
         duty, duty tour, tour]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "tour of duty":
   anchor watch, bit, day shift, dogwatch, full time, graveyard shift,
   half time, lobster trick, night shift, overtime, part time, relay,
   shift, split schedule, split shift, stint, sunrise watch,
   swing shift, time, tour, trick, turn, turn of work, watch,
   work shift