Convert datetimes to a comparable value in an expression.

def convert_values(self):
        """Convert datetimes to a comparable value in an expression.
        """
        def stringify(value):
            if self.encoding is not None:
                encoder = partial(pprint_thing_encoded,
                                  encoding=self.encoding)
            else:
                encoder = pprint_thing
            return encoder(value)

        lhs, rhs = self.lhs, self.rhs

        if is_term(lhs) and lhs.is_datetime and is_term(rhs) and rhs.is_scalar:
            v = rhs.value
            if isinstance(v, (int, float)):
                v = stringify(v)
            v = Timestamp(_ensure_decoded(v))
            if v.tz is not None:
                v = v.tz_convert('UTC')
            self.rhs.update(v)

        if is_term(rhs) and rhs.is_datetime and is_term(lhs) and lhs.is_scalar:
            v = lhs.value
            if isinstance(v, (int, float)):
                v = stringify(v)
            v = Timestamp(_ensure_decoded(v))
            if v.tz is not None:
                v = v.tz_convert('UTC')
            self.lhs.update(v)