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.
Technically, seconds should be allowed to go as high as 61, even if you don't like denormal times; the relevant time standards provide for up to two leap seconds at any given half-year.