msgpack_datetime
https://sr.ht/~nabijaczleweli/msgpack_datetime
Unpack Elixir DateTimes from msgpack extension -1 timestamp fields

All three formats described in
  https://github.com/msgpack/msgpack/blob/master/spec.md#timestamp-extension-type
are supported for packing and unpacking (timestamp 32/64/96).

msgpack only tries the ext callbacks for tuples
(which is how Erlang structs are implemented) but packs maps (Elixir structs) as maps.
Thus, to pack, DateTime.now_utc(), you'd pack it as {MsgpackDateTime, DateTime.now_utc()},
and you'll get back {MsgpackDateTime, %DateTime{}} in return.
Example:
  dt = ~U[2025-12-10 12:53:25Z]
  :msgpack.pack({MsgpackDateTime, dt}, [{:ext, MsgpackDateTime}])
  === <<214, 255, 105, 57, 109, 69>>
  :msgpack.unpack("\x93\xC4\x06record\xD6\xFFi9mE\xCD\x01\xA4", [{:ext, MsgpackDateTime}])
  === {:ok, ["record", {MsgpackDateTime, dt}, 420]}

def deps do
  [
    {:msgpack_datetime, "~> 0.1"}
  ]
end

Reproduce test data with:
  apt install libmsgpack-cxx-dev
  make
  ./testdata [dir]