def _check_packers(self):
        """check packers for binary data and datetime support."""
        pack = self.pack
        unpack = self.unpack

        # check simple serialization
        msg = dict(a=[1,'hi'])
        try:
            packed = pack(msg)
        except Exception:
            raise ValueError("packer could not serialize a simple message")

        # ensure packed message is bytes
        if not isinstance(packed, bytes):
            raise ValueError("message packed to %r, but bytes are required"%type(packed))

        # check that unpack is pack's inverse
        try:
            unpacked = unpack(packed)
        except Exception:
            raise ValueError("unpacker could not handle the packer's output")

        # check datetime support
        msg = dict(t=datetime.now())
        try:
            unpacked = unpack(pack(msg))
        except Exception:
            self.pack = lambda o: pack(squash_dates(o))
            self.unpack = lambda s: extract_dates(unpack(s))
check packers for binary data and datetime support.