You need to talk to them about it and find out why the change. Then decide if you want to continue with them or tell them it won't work.

I am open from 7:30-5 and families are allowed to use any amount of that time that they need. All my families are on a regular schedule though so I always know when to expect everyone. I just ask if they are needing to change their time on any day that they let me know in advance - usually the day before.