The above condition will disable the user next day. If you want the user to be disabled exactly at 6 PM then you would have to modify the condition and consider below scenarios:
1. The user update rule needs to be marked as detective and the DetectiveProvisioningRules job should be scheduled to run frequently.
2. In your advanced query condition, you need to add 18 hours to the user's enddate and the difference of that with currentddate should be greater or equal to 0.
3. Consider the timezone of the user's location.
Please use these suggestions to form your condition for the user update rule.