I seem to be a little confused. Your system used to run 24 hours a day. Now it is turning after 23 hours and 15 minutes, or it is shutting down at 11:15 P.M.?
The only thing I can think of that has an expiration date is a lease on a network connection. Don't know why that would shut the system down, unless some type of conflict was occuring.
Any parental controls involved?
There are some performance troubleshooters, you might try one of those.
Hopefully, you will find something in the Task Scheduler as Kemical suggests.