The Julian (old) calendar was developed during the reign of Julius Caesar, in the first century B.C. It was based on the assumption that a year is 365.25 days long, so the calendar had 365 days with a leap year every four years. Unfortunately, the year is actually about 365.2422 days long, so the calendar was losing about 11 minutes a year, or one day about every 128 years. By AD 325, when the First Ecumenical Council at Nicea met, the actual spring equinox (the day when night and day are of equal length) was falling on March 21 instead of the original March 25. At that Council, the Church decided to accept the Julian calendar with its acknowledged flaws and to set the date of the equinox (for the purposes of determining the date of Pascha) at March 21.

Eventually, in the 1500s, it got to the point that the equinox was falling on March 11, and people realized that if things went on this way, they would eventually be planting crops in January. So Pope Gregory VII commissioned a group of scholars to correct the calendar. This they did by deciding that on three of every four turn-of-the-century years, there would be no leap year. This gets the annual discrepancy down to an average of about 20 seconds, so the calendar should be accurate for the next 40,000 years. In order to get back into synch with reality to begin with, in 1582 they just jumped from August 5 to August 15, dropping ten calendar days. (The Julian calendar is now thirteen days behind the Gregorian because it had leap years in 1700, 1800, and 1900, when the Gregorian calendar did not. Both calendars had a leap year in 2000.)

So far, so good. Nobody had any problem with the accuracy of the new (called Gregorian after Pope Gregory) calendar. Unfortunately, when Pope Gregory informed the Orthodox East about the new calendar, he made it into a package deal with a set of theological changes-things like purgatory, selling indulgences, the filioque, etc.-that were completely unacceptable to the Orthodox Church. The Orthodox hierarchs could not find any way to accept the new calendar without leading people to believe that they were endorsing all the theological changes along with it. So they said, "Nope, sorry, we're sticking with the Julian calendar." The calendar wasn't a theological issue in itself; this was just the only possible response to the ecclesiastical/political situation at that time.

Again, so far so good until 1923. Then Patriarch Meletios of Constantinople called a council to discuss putting the Church onto the Gregorian calendar. At this time the political situation was again very precarious, with Greece trying to win independence from Turkey. Unfortunately, the council was not truly representative of world Orthodoxy. Some people didn't come at all, and others left when they realized what Meletios's agenda was. It seems that he was trying to repeat history and again to package the Gregorian calendar with a lot of Western innovations-so that he could get financial and political support from the Anglican and Roman Catholic churches. Ultimately a council of only nine men decided to adopt the Gregorian calendar for the whole Church.

Now, because of the decision of the Council of Nicea to accept the Julian calendar, a new calendar could only properly be accepted by an ecumenical council or a properly representative local council, which this was not. So because of the improprieties involved in the decision, the Patriarchs of Moscow, Jerusalem, and Antioch rejected it. (Antioch later changed its mind and accepted the new calendar.) They had no problem with the calendar itself, only with the way the decision was made. There needed to be an ecumenical council to decide the issue, but what with the Russian Revolution, etc., that wasn't possible at the time.

So that's why the Jerusalem Patriarchate-in company with 90 percent of the Orthodox world-is on the old calendar. The calendar is still not a theological issue, nor should it be a sort of "litmus test" of true Orthodoxy, although this attitude is occasionally encountered in some old calendar proponents. It's simply the result of some unfortunate historical developments.