@staticmethod
def _validate_date(date):
    """validate date has format of YYYY-MM-DD"""
    date_list = date.split('-')
    if len(date_list) == 3:
        if len(date_list[0]) == 4:
            if len(date_list[1]) == 2 and len(date_list[2]) == 2:
                return True
    raise Exception("format should be 'YYYY-MM-DD'")
