@staticmethod
def delta_days(date1, date2):
    """
        return days of (date2 - date1)
        """
    if type(date1).__name__ == 'LunarDate':
        date1 = date1.to_datetime()
    if type(date2).__name__ == 'LunarDate':
        date2 = date2.to_datetime()
    return date2 - date1
