Fix cronjob month calculation

Fix cronjob month calculation

The current calculation ignores given months, this is fixxed with this pull request.
To detect any issues with the new implementation i'll run several tests without any differences with the currently used cronjobs used by the wsc.
https://gist.github.com/Morik/27804aaa57916150dd08268cd8e224d8
Running those tests took quite a while but there have been no differences for all calculations without a month selection so this fix should have no problems with them.
It also allows some exotic cronjobs like the 29. February etc.