[tests] Fix test expectations

Sometimes, I’m just stupid.

I knew that relying on ‘now’ was going to cause issues with calculations related to hours in some cases.

I knew it and should have fixed this before.
Remove a not-so-useful helper method

This change also removes the related unit tests.
[fix] Tweak interval calculation

This fixes a few issues related to rule #2. Till now, this was not working under some circumstances.
[tests] Add placeholder test

Just for the sake of completeness. Of course, this should never be done on a real-world project since empty tests are useless by definition.