[Hostage]

The Old Calendar (Julian Calendar) has his birth on January 7.

The Julian Calendar predates the Gregorian Calendar (from the 16th century, when Pope Gregory XIII instituted the New Calendar). The Gregorian Calendar set the Nativity at December 25. The thirteen-day change was to align the Old Calendar with the civil calendar; purely administrative and scientific purposes.

There was a battle over the calendar between Roman and Eastern Catholic. The Gregorian calendar became a frontline of battle to make the Orthodox submit to the RCC.

Here is a very good background to understand the history and the science behind the change:
http://www.copticchurch.net/topics/coptic_calendar/nativitydate.html

[Campion]

No, that's not right. Christmas is December 25th on both calendars, but December 25th on the Julian calendar is (currently) January 7 on the Gregorian calendar. The Gregorian calendar reform didn't "set the nativity at December 25," but it removed 10 days (at the time) from the calendar to put the calendar's relationship to the seasons back where it was at the time of the Council of Nicaea.

Since then, the two calendars have continued to diverge. The difference is now up to 13 days, and will go to 14 days in 2100.