I propose that making the position of hour, minute and seconds the canonical definition is worse than using their names.

So this: { type: "hms", hour : 10, minute : 45, second : 16 }

is better than this:

  ["hms", 10, 25, 16 ]

Either go across the wire fine. The names are not just local names, they describe the meaning of the data.

In the JavaScript world we would call the map an object.