Could you please help us with few more  details 

Are you talking about  surfboard central app or the surfboard manager app?

Is this a parental control setup?

You can also give it a try on modem interface or GUI.

This is in my surfboard app under my experiences/parental control. I set a custom time (Sun-Thurs 2200-0700) and the app will not save it. The app changes it to a completely different time like 2200-2359.

Please note to customize the time on the Surfboard central app under my experiences/parental control follow  2 rules one for 22:00-23:59 and another for 00:00-07:00 and save it.