Whoever thought supporting sexagesimal numbers was a good idea needs to spend some extended time away from their computer to reflect on what they’ve done
IMO anything other than the basic types supported by JSON (number, true, false, null) ought to be be parsed as a string. Or if you really insist, some kind of special syntax to make it clear it's not a string would probably be acceptable.
fastest_mile: 2:54
Not saying it's a good idea, mind you, but at least it's an ethos.