laterthan fix and acceptance tests

Current state makes it useless.

Redmine: https://dev.cfengine.com/issues/6530

Please backport to 3.6.x for inclusion in 3.6.2.
While semantically correct, I find that very confusing. Since this is policy language, I think we should opt for easy to read (and by easy, I mean like you would find it written on a calendar, except for being zero-based).