First off to get started I create a couple of SharePoint-Lists:
The patch-plan holds a list of all servers that need to be patched as well as the number of days after "patch-thuesday" they are supposed to be patched (PatchOffset).
The PatchCalender will actually hold all the dates for the servers to be patched.
In order to populate the PatchCalendar I create a new site-workflow. At first I'm querying the PatchPlan list get all the servers to be patched into a collection. Next up I create a collection with all the months, to later on create monthly patching dates.
So next I can loop over the collection of months and compute the second tuesdays of the month.
Finally I loop over the list of servers queried at the beginning and create calender-entries for each server.
This way - with starting the site-workflow, I can populate the calender with all the patch-dates for the given year.