unit : SECOND
                | MICROSECOND
                | MINUTE
                | HOUR
                | DAY
                | MONTH
                | YEAR

def p_unit(self, p):
        """unit : SECOND
                | MICROSECOND
                | MINUTE
                | HOUR
                | DAY
                | MONTH
                | YEAR
        """
        unit = TimexLexer.reserved_words[p[1]]
        unit = unit.lower()
        p[0] = unit