My iPhone crashes and rebots ofter I change date and time

Hellow I need some help when i change date and time my iphone 3 gs crashes after 1 min wont let me unlock it, if i try to call someone just rebots, or just rebots whitout doing anything, its not a batter problem because i just changed it, at first i thought it was a ram problem but i think its best to ask experts

Plz any advice its welcomed