Restrict Previous Date Calendar Booking

I am facing difficulty in implementing the functionality in ZOHO Creator. When the user clicks on the Previous Date on the calendar, the System should Alert "Booking Not Allowed in Previous Dates" and the system should not open the Booking form for previous dates.

Same functionality I also for want for specific User type.

For example, only the admin can do bookings on previous dates.

I have tried this, but not successful:

if Date_Field < zoho.currentdate { alert "You cannot book in Previous dates." }

Can you please help me to solve this problem? I am also facing difficulty while putting the limit on bookings. Let's say I have opened a class booking for 15 students, how can I keep an eye(functionality) that each time a student makes a booking and system counts and displays the remaining available bookings. And when the limit reaches system should stop bookings for a specific date and class.