Updates information about the NepDate

def update(self):
        """ Updates information about the NepDate """
        functions.check_valid_bs_range(self)
        # Here's a trick to find the gregorian date:
        # We find the number of days from earliest nepali date to the current
        # day. We then add the number of days to the earliest english date
        self.en_date = values.START_EN_DATE + \
            (
                self - NepDate(
                    values.START_NP_YEAR,
                    1,
                    1
                )
            )
        return self