In theory the difficulty can change before the 15th of September. We agree to you that this will NOT happen, for sure.
However, normally we wait until the resolution date to resolve a event. But in this case we take the risk and resolve the event indeed earlier.
(in the VERY unlikely case that the outcome of the event will change we will pay the different outcome ourselves)
It is better to say, "the next difficulty level" rather then a specific date because the difficulty might not change on a specific date. It is not fair not to close "the september 15" prediction down since the difficulty level just happened on September 13. It is not fair to the users and create confusion--unnecessary loss of money for nothing.
This is no different then asking people to predict the difficulty level for August 31st 2014. The event already had happened so if you know what you are doing you just look up. It is the one that does not know that is going to get hurt.
I think it is fair for you to cancel the bets placed after the most recent difficulty level which is September 13.
