How to stop date time field to select Post dates

Above is the custom entity created my client want me to stop post dates he only needs today and previous date and time  how i will stop post dates ...

there are three fields 

date visited..field  this date should auto populate to start time and  end time fields

but the end time()not date   ... must not be equal to start time() 

date must be same in start and end field but the time must be different 

need help