Many "dogmatic" Old Calendarists (henceforth OCs for short) identify the New Calendar that the Church of Greece and New Calendarists in general (henceforth NCs for short) follow with the Gregorian/Papic/Frankish Calendar, whereas the Old (Julian) Calendar they themselves follow they consider it to be the traditional genuine Ecclesiastical Calendar. But that is not the case.
The New Calendar which we follow is not Gregorian, Papic or Frankish, as they call it, but Julian. And here is the proof:
Firstly: If we open any high-school textbook on cosmography we will see on the subject of calendars the difference between the Gregorian and Julian Calendars from a purely calendarist point of view
In other words, we know that one year is made up from precisely 365.242217 solar days. The length of the civil year based on the Julian Calendar is 365 days and 1/4 of a day, because every 4 years one day is added, namely the 29th of February. Therefore the Julian Calendar exceeds the true year, i.e. is greater than the true year, by 365.25 - 365.242217 = 0.007783 solar days. This difference within a period of 400 years comes to 3.1132 days. Therefore the true date falls short by 3.1132 days every 400 years. In order to correct this error, Pope Gregory XIII, with the aid of the astronomer Lillio, ordered the day after the 4th of October 1582 to be called 15th of October and not 5th of October. In order that this error would not be repeated in future, he also ordered that in a period of 400 years they should not take 100 leap years, as is done by the Julian Calendar, but only 100 -3 = 97. Thus, according to the Gregorian Calendar, in a period of 400 years the date falls short now only by 0.1132 days.
4,000 years must pass before the date falls short by 1.132 days.
In order to correct the Julian Calendar, Pope Gregory ordered that the years of the centuries (i.e. 1600, 1700, 1800) are not leap years unless the number of hundreds is divisible by 4. Thus, the year 1600 was a leap year according to the Gregorian Calendar, because the number of hundreds 16 is divisible by 4; according to the Julian Calendar it is still a leap year because the whole number 1600 is divisible by 4. However, the years 1700, 1800, 1900 are leap years according to the Julian Calendar, because these numbers, 1700, 1800, 1900, are divisible by 4, whereas they are not leap years for the Gregorian Calendar, because the numbers of the hundreds, namely 17, 18 and 19, are not divisible by 4.
Thus, after 1600 years that have passed approximately since the First Oecumenical Synod, we the NCs, since we follow the Julian Calendar and will have as leap years those years that are divisible by 4, we will have a new difference of 13 days, whereas the Gregorian Calendar will not have such a difference, because every 400 years it will have 97 leap years and not 100.
Here is the difference between the New Calendar and the Gregorian Calendar, from a calendarist point of view. They will have 97 leap years every 400 years, we will have 100
However, Pope Gregory did not stop at this calendarist modification
. He also adjusted his Easter upon his calendar, not taking into account the Passover of the Jews and the decision of the First Oecumenical Synod regarding the Christian Pascha and the Jewish Pascha.
Page 9 of "the Rudder" of our Church, there are four things that must be satisfied for the celebration of Pascha:
a) Vernal equinox
b) First full moon after this equinox
c) After the Nomicon Pascha
d) the first Sunday after a) - c).
The Gregorian or Papic Calendar takes into consideration only a), b) and d). It totally ignores c), the legal pascha of the Jews. Thus Papics can happen to celebrate their Easter together with that of the Jews or before the Jewish Pascha despite the 7th canon of the Apostles and despite the historical fact that Christ resurrected after the Pascha of the Jews. However, we NCs take into consideration all 4 points on the Christian Pascha, namely a) - d), and thus differ from the Gregorian or Papic Calendar not only from a calendarist but also from an ecclesiastical point of view. In other words, we celebrate our Pascha after the Jewish Pascha, in accordance with the decisions of the First Oecumenical Synod.
Therefore we conclude that from a Calendarist and an Ecclesiastical point of view both we the NCs and the OCs differ from the Gregorian Calendar.
Both NCs and OCs follow the Julian Calendar from a Calendarist and from an Ecclesiastical point of view! Why then do some OCs identify the New Calendar, the one we follow, with the Papic Gregorian, since there is such a great difference from a calendarist and an ecclesiastical point of view between the two calendars?
We guess their objection: "Why promote the moving forward of the 13 days?"
To which we answer: This promotion by itself does not violate the Julian Calendar, but only the calendar almanac
of this Calendar. Our Calendar was and has remained Julian. The OCs have Julian. So do we. Only that our Julian Calendar is corrected, adjusted; this is why it is commonly referred to as revised Julian. Why though alter the calendar almanac of the Julian Calendar?
We know from the Rudder of our Church that the Fathers of the First Oecumenical Synod that established the Paschal celebration based on the vernal equinox had vernal equinox then on the 21st of March. This we know from the Rudder (see p. 9). After the passing of approximately 1600 years however from then till today, we deviated from that date with the result of having vernal equinox on the 8th of March. This is also mentioned in the Rudder (p. 9). With a "leap" of 13 days thus, we, the so-called "New Calendarists", have as vernal equinox the one the Holy Fathers of the First Oecumenical Synod had. This way we have the old date of the vernal equinox, the 21st of March, whereas the OCs have the new date of this equinox, namely the 8th of March. Therefore, rather ironically
the NCs follow the old date, whereas the OCs the new date. We follow the old calendar almanac, the OCs follow the new calendar almanac. We are based upon the old Julian Calendar of the First Oecumenical Synod, the OCs are based upon the new Julian Calendar.
As a consequence, the so-called NCs are in reality old calendarists and the so-called OCs are in reality new calendarists!