Mesoamerican calendars was of ritual significance to a number of Mesoamerican cultures. A third major calendar form known as the Long Count is found in the inscriptions of several Mesoamerican cultures, most famously those of the Maya civilization who developed it to its fullest extent during the Classic period (ca. 200–900 CE). The Long Count provided the ability to uniquely identify days over a much longer period of time, by combining a sequence of day-counts or cycles of increasing length, calculated or set from a particular date in the mythical past. Most commonly, five such higher-order cycles in a modified vigesimal (base-20)
Adoption of the Gregorian calendar to use their own calendars, which usually result in fixed dates being celebrated in accordance with the Julian calendar. This is most interesting in the case of the Syriac Orthodox Church, as one of its Patriarchs Ignatius Nemet Allah I was one of the nine scholars who devised the Gregorian calendar. The Indian Orthodox Church uses the Gregorian calendar along with their autonomous Syriac Orthodox counterparts in India, the Malankara Jacobite Syriac Orthodox Church. The Armenian Apostolic Church adopted the Gregorian calendar in 1923, except in the Armenian Patriarchate of Jerusalem, where the old Julian calendar is still in use.