never a comma before a "and" (yes, it's grammatical, but still about the kata)

The phrase here is a list. In my experience, lists are separated by commas if there are three or more parts to it. (Serial comma). This isn't really an issue about this kata, but relates to grammatical rules.

¯\_(ツ)_/¯

 "1 hour and 1 second"
        ^
 "1 year, 11 months, 4 days, 8 hours, 10 minutes, and 42 seconds"
                                                ^

This is inconsistent/not logical (from the description).

A justification doesn't mean it's good/acceptable. The kata is about creating human friendly output. This is not human friendly output.

I found it also annoying, but the description says:

Commas appear only if there are three or more units in the phrase.

 "1 hour and 1 second"
        ^
 "1 year, 11 months, 4 days, 8 hours, 10 minutes, and 42 seconds"
                                                ^

This is inconsistent/not logical (from the description).

Duplicate/very similar to this kata: https://www.codewars.com/kata/52742f58faf5485cae000b9a

Fixed. Added note in description to say that they can be equal.

end is always greater than start

No, start and end may be the same.

Fixed.

This solution is clearly wrong as can be seen by testing it with inputs (1,2,3) (expected 0, produces 1) and (1,-2,-3) (expected 0, produces 1). Nevertheless, it passes the test suite every last time. Might I suggest adding an element of randomness into the testing so that c is not always 1 or 0?

C++ Translation

beginners struggle

possibly actually reading the description is a novel idea for them. but that's no reason to have this kata; any other kata will do.