The problem here is that the main attraction of these games is how fast they work.  There's no blockchain timestamp until the transaction is in a block, which takes on average 10 minutes to happen.  You don't want to have to wait for 10 minutes on average each time you bet before you find out whether you won or not.

If you're worried about the operator cheating, don't play in the hour before midnight (server time).  This should remove the ambiguity.