This failure solution in this case:

Test.expect(elapsed_seconds((Time.now - 30), Time.now) == 30)

Fair enough, guess I was overthinking

The Kata description explicitely states: "The start time will always be before the end time."

Also AM/PM has nothing to do with this. "Time is stored internally as the number of seconds with fraction since the Epoch, January 1, 1970 00:00 UTC"

I don't think this would work if there were more test cases ._.
First instance of an AM/PM where the start_time >= than the end_time for example

oh dear, and to think it was just this easy all along XD