hi ncustod,
I haven't tried, but there is an authentication source called "[Time Source]" in ClearPass. You should be able to create a new attribute, which will give you the date of today and the time of 5pm.
During authentication, you can use this time in the enforcement profile to update the Expire-Time-Update field. but be aware, you can only expire the account sooner. you cannot extend the lifetime of the account. So make sure, that the account is valid for at least this day 5am.