A few points for the non-Orthodox or Orthodox but confused among us <G>:
* THE "OLD CALENDAR". The "old calendar" is simply the Julian Calendar in use in most of Europe til the "new calendar" (the Gregorian Calendar) was adopted by the Catholic church and, later, most of Europe and its colonies. Most Orthodox Christians use ("are on") the old calendar for church feasts, fasts, etc. In the early 20th century, a synod of several national Orthodox churches agreed to adopt a calendar that is usually called the "Revised Julian Calendar". It retains the traditional Orthodox dates for Pascha and associated holidays, but otherwise closely matches the Gregorian calendar. Romania, Greece, the U.S., Finland and a few other countries use the modified Gregorian/New Julian "New Calendar". Russia, most of Eastern Europe, and most Arab and North African Chalcedonian Orthodox Churches use the Julian "Old Calendar". These groups commune with one another and recognize the others as parts of the Church. Both groups are Orthodox, not Catholic.
* THE "OLD CALENDRISTS". Some Orthodox Christians reject a number of changes that the larger Orthodox churches have made since the early 20th century in the calendar, but also with other aspects of church practice and discipline. These are usually what is meant by "old calendrists". Some of them are in communion with the large national Orthodox churches. Some are not. The ones that are not in communion with the large national Orthodox churches may not recognize the large national Orthodox churches as Orthodox or parts of the Church. The large national Orthodox Churches, in turn, generally view the traditionalist "old calendar" churches that reject communion with them as schismatic. But both sides are Orthodox, not Catholic.
* THE "OLD BELIEVERS" (AKA "OLD RITUALISTS"). In Russia, during the reign of Peter the Great, the patriarch Nikon ordered certain changes to be made to church discipline and practices. A fairly large number of Russian Orthodox (25% or more) refused to go along, and broke from the Russian Orthodox Church over these changes. Like the "old calendrists", the "old believers" are traditionalists who refused to accept changes ordered by the bishops of their church, but the schism occurred during the middle and late 1600s instead of the early and middle 1900s. One group of old believers reunified with the Russian Orthodox Church Outside of Russia (ROCOR) while it was still out of communion with the Moscow-based Russian Orthodox Church, and thereby ended up back in communion with the main Russian Orthodox Church when ROCOR and the ROC reconciled in 2007. Most "old believers" are still not in communion with the ROC, and the ROC and most other Orthodox churches view them as schismatic. They are Orthodox, however, not Catholic.
The Catholic Church (i.e. all traditional Christian churches that recognize the Roman Pope as their supreme leader on earth) has its schisms and its problems, but these particular schisms and any problems associated with them are our own. Unfortunately. <sigh>
Lord, have mercy!