@The92Ghost Opening hours are not a setting, but just something you manage yourself. It's been some time, but if I remember correctly you can actually go to the store panel(while not on the vet lot) and open it to run. You have to later close it from the panel as well. And you can go to the lot open the clinic, play as a vet or otherwise be on the lot, and then close it when you wish.
Myself would chose days and hours I'd open the clinic, like Mon-Fri, I'd open it at 8am and close it when it was 5pm. It's something you manage manually instead of a specific setting.