Avoid Google Calendar Overbooking

Hi guys,

I’m new to AppSheet so probably (and hopefully) the challenge I’m facing here should be simple!

I’m building an app to help a dentist to have her patients book a consult. However, it cannot allow overbooking. Physics, you know? She can’t be in two place at the same time. So when her patients try to book a consult during a period of time that she have already a appointment, I need to notify the conflict to the user, and of course stop to appointment creating.

As I am pretty new to this, I’ll need to know in which column I should right the valid if expression that you guys will come up with.

If I’d be so bold, I will also need to set the clinic’s work hours (14:00-20:00 from Monday to Friday). I mentioned that, 'cause I figure this should impact the expression, right?

I’d really appreciate if you guys could help me with that.