Orthodox Old Calendarists calendar support (Julian calendar)

I believe you should have different issues for different calendars to be supported. This way you can have people that are interested in a specific calendar jump in and offer their help either by pointing to the respective algorithm or by providing patches or by beta-testing newly added calendars.

To that end, I am creating this issue for supporting the calendar used by the Orthodox Old Calendarists:

...The term Old Calendarist refers to any Orthodox Christian or any Orthodox Church body which uses the historic Julian calendar (called "Old Style Calendar" or "Church calendar" or "Old Calendar"), and whose Church body is not in communion with the Orthodox Churches that use the New Calendar. ...

Here's another related article from the OrthodoxWiki:

... Old Calendarists are Orthodox Christians in Greece, Romania, Cyprus, Bulgaria that oppose the use of the Revised Julian liturgical calendar (the Gregorian calendar combined with the Orthodox Paschalion) in the early 20th century. ...

I am not an Old Calendarist myself, so I can't actually help more than providing what I already know. You see, I am Greek and we do have a part of Orthodox Old Calendarist Christians here like many Eastern Orthodox countries/Churches do. We are talking about a considerable amount of people using it.

The reason I am proposing this calendar is because I believe it will be really easy to be implemented, since the only difference from the Gregorian calendar is that it is 13 days behind.

Thank you for considering this one and for all the effort and time spent on the module.