Fixed T-N cycle offset bug for n > 24 (HoursOfTheDay cycling)

Example:
[[[0]]]
    graph = "foo[T-36] = bar"
This graph results in bar triggering off foo at T-36; but it is also
used to infer that one of the valid cycle hours of foo is 0z minus 36,
or 18z. Conversion of T-N back to the 0-23 range was broken for N > 24
with the result that foo could sometimes spawn into the wrong cycle.
No change - to NOTE that previous commit message contains an error.

(replace 36 with 30 - this was just a typo in the commit message).
Merge branch 'BugFix.HoursOfTheDay-cycling' into 487.future-task-triggers

Conflicts:
	lib/cylc/cycling/HoursOfTheDay.py