there are provisions to make sure you can't win on time. If a player is winning (or for that matter if he feels the game is a dead draw.) but is running out of time he can ask the arbiter to declare the game drawn (he may not ask for a win). The arbiter may either
A. declare the game drawn if it is clearly a draw or a win for the requesting player.
e.g. black may request a draw
B. add a delay to the time controls. (a delay is a set amount of time that is renewed each move but anything left does not carry over. A players clock only starts to run down after the delay has expired. usually 2-5 seconds.)
C.give the requesting player a set amount of time per move in which he must prove he has a draw. (i.e. 10 sec per move)
D.decline the request if he feels the position does not merit intervention
method C is usually the most popular among players for obvious reasons.