Resolving then.

function sum_intervals(array $intervals): int {
  return 1;
}

That doesn't return the correct answer (obviously) but doesn't throw that error. Are you sure your function's name is ok?

You can't delete it, just leave it.

the duration is expressed as a combination of years, days, hours, minutes and seconds.

There is no months there, so no mention is needed at all.

Wh do you ask this in Cheat Detection...!??

1. the tests you see are sample tests. When you click "attempt", there more of them
2. yes, that's the usual/normal way to do it.

I tried several solutions and they worked fine. There is sommething wrong in your code. It seems that the missing element is the last.