Douglas Harper's Etymology Dictionary
Wiktionary
short-timer
n. 1 (context military English) A soldier during the Vietnam war who had less than two months to serve. 2 (context military English) A person who would not be required to do a difficult or dangerous task. 3 (context colloquial English) An employee who has give notice of their intent to leave.
Usage examples of "short-timer".
A few short-timers, men and woman in for one-day safaris, watched the screens intently.
You go slumming for short-timers, working guys you normally wouldn't look at, that much I can understand.