Time (big) problem or bug

Hello, i’ve found a problem with the time of matches and the limit for the users to enter the predictions.
The thing is that i put the limit to 5 minutes before the game starts, but if the time’s reached the limit a user can’t enter prediction for that match. Until here everything is fine, but if a user change the time or date from its computer the prediction forms roll back to active to enter the prediction.
Somebody have a solution for this?
Thank you in advance