Restrict Previous Date Calendar Booking

Restrict Previous Date Calendar Booking

Hi All.
 I am facing diffculty in implementing the functionality. When User click on Previous Date on calendar , System  should Alert "Booking Not Allowed in Previous Dates" and system should not open Booking form for previous dates.

Same functionality i also for want for specific User type.

For example, only admin can  do bookings in previous dates.

I have tried this:

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