Greg,
At this time, we do support time frame since the password is generated or when it is use for the very first time. this is a configurable option under Configuration :: Guest Access :: Guest Pass Generation.
If you only need a WLAN in a certain period of time, you can always use our schedule features. For example allowing the use of the WLAN only during working hours or during an specific period of time, for example from noon to 12:45.
Victor,