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 think we should leave high limits for the ones other than Years; there are times when "denormal" dates and times are useful (and this is why mktime's parameter is non-const; it normalises denormal dates); for example, being able to express "seventy hours from now" as the present date-time with 70 added to its hours field, without any obligation to fold the result mod 24, spilling the surplus into the days field and doing the complicated mess of things to checking whether that now overflows the month field.