we all have heard one reason Washington crossed the delaware on christmas eve was that many of his troops were quitting after new years eve. so why can't I throw caution to the wind in december, too?

In my campaign it seems I get troops in Feb and August, so they should leave in jan and july, but I can't tell who is leaving when -- am i missing something?
if i am not, i think it should be noted somewhere, obviously the real-life leaders new when the different units' contracts were going to expire
thanks
great game
after i finish my campaign i will find a pbem game i think