>>> import datetime
>>> def validate(date_text):
    try:
        datetime.datetime.strptime(date_text, '%Y-%m-%d')
    except ValueError:
        raise ValueError("Incorrect data format, should be YYYY-MM-DD")


>>> validate('2003-12-23')
>>> validate('2003-12-32')

Traceback (most recent call last):
  File "", line 1, in 
    validate('2003-12-32')
  File "", line 5, in validate
    raise ValueError("Incorrect data format, should be YYYY-MM-DD")
ValueError: Incorrect data format, should be YYYY-MM-DD