Validation of Age from date of birth.

[Jared Abong'o]

I need two things,

  One can't enter a future date of birth and when entered your birthdate it will autopopulate your age . 

I would like to know how to validate doctor's booking , one can't book the same doctor twice or at the time for example Dr.X has an appointment at 10-11pm . If i want an appointment at the same time it will give me a warning that is he booked . Kindly help.