from typing import Tuple
import struct


def split_timestamp(packed: bytes) -> Tuple[bytes, float]:
    """Split out the value and timestamp from a packed item.

    The item contains 8 bytes with the timestamp in big-endian IEEE-754
    double precision, followed by the value.
    """
    assert len(packed) >= 8
    timestamp = struct.unpack('>d', packed[:8])[0]
    return (packed[8:], timestamp)