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.
I tend to agree with @kacfengine here; even though it is a valid consideration to allow large and negative dates, I would expect that more than 95% of the usage would be calendar dates (patch after date T). There is also value in helping the user detect accidentilly typing an unintended date, e.g. by switching the hour and day parameter.

The zero-based thing is really bad, though, I think (month 0 or day 0??). But this could break backward-compatibility, so definitively not for 3.6..