https://en.wikipedia.org/wiki/Mount_Athos#Date_and_time_reckoning
The Julian calendar, currently having a difference of 13 days from the Gregorian calendar, is still used on Mount Athos. In 1923, as a means to eliminate the divergence existing between the religious and civil dates, after a synod in Constantinople, part of the Eastern Orthodox Churches dropped 13 days and adopted the Revised Julian calendar, which is synchronised with the Gregorian calendar, at least until 2800.[note 2] However, although under the direct jurisdiction of the Ecumenical Patriarchate of Constantinople, the spiritual head of the monastic state, nearly all the monasteries of Athos refused to follow the revised calendar and finally, for the sake of uniformity, the patriarch asked the single monastery that used the revised calendar to revert to the Julian.
Also in use is Byzantine time, in which the day commences at sunset as does the liturgical day[note 3] and not at midnight as in the reckoning of civil time, and the difference between the two varies according to the season of the year. Because the time interval from sundown to sundown is not constant, clocks showing the Byzantine time require continual manual readjusting which in current practice is done weekly, on Saturday, if the sky is clear; where the summit of Athos is visible, 12:00 is set when the last rays of sunlight cease to shine on the tip. Some monasteries also have a clock showing civil time since boat schedules run thereon (and on the civil calendar) as well as for pilgrims who may be disoriented by Byzantine time reckoning.
