A POSIXct or POSIXlt or anything that can be coerced to a numeric.
seconds
The number of seconds since 1970-01-01 00:00:00 UTC. Can be negative. Don't use seconds and nanoseconds if you use posix (and vice versa).
nanoseconds
The number of nanoseconds since 1970-01-01 00:00:00 UTC. Must be less than 1,000,000,000 and greater than 0.
Value
A serialized timestamp that can be used as input to msgpack_pack. Briefly, this is an extension type -1 that is variable length, depending on the desired range and precision.