The latest rumblings are all about how the IMF won't be paying out a dime more to Greece unless it gets its act together and resolves this referendum issue in one way or another.
The question is: How long does Greece have to pass the referendum, get the money, and pay off its debts.
This chart should answer that.
Sent to use from Erwan Mahe of OTCexGroup, it shows when Greece has some big debt payments due. Obviously, you can see why Greece can't wait until next year.
12/29 looks like a pretty certain drop dead, if not before that.