How to disable past dates in datepicker for specific doctype

For a specific doctype, I want to disable selecting past dates in the datepicker. How can I do that?

Make the field read only and Today in Default field.

If I your suggestion follow your suggestion, it will make the datepicker disabled. then the datepicker will not popup, when I click that field.
I want user to select the current date and any future date only.

did you find a solution?

i also need this. any solution suggest please.