Motivation: I've seem multiple solutions where Clock.__add__ either (1) returns a str or (2) modifies and returns self. Ideally a = b + c should not mutate b or c and Clock + int should return a Clock.
Proposal: Add a test or two where,
now = Clock(0, 0)
future = now + 5
test.assertNotEqual(now, future)
test.assertEqual(future, Clock(0, 5))
Possibly replicate for __sub__ as well.
I'd be happy to work on this myself.