The problem with the new calendar innovation, is two fold.
a) The New Calendar is materially the Gregorian Calendar. It is a sophism to say it is not the Gregorian Calendar, but simply the "revised Julain Calendar." The EP's Patriarchal Synodical of 1920 (which introduced this innovation) explicitely states the purpose of the new calendar, which is to cause the Orthodox calendar line up with the secular/Papal calendar. Thus, it is a difference that really is no difference.
b) The above is a problem, since three Orthodox synods (whose acceptance diffused throughout the entirity of the Orthodox world), which the EP itself presided at (meaning at the
very least that their canons are obliging for the EP and it's dependencies - though, their pan-Orthodox acceptance makes their canons actually universal in character) not only rejected the Gregorian calendar, but actually
anathematized it and those who may want to adopt it, results in a difficult, but unnavoidable conclusion -
the EP fell under it's own anathema in 1920. On this basis alone, the Old Calendarists are vidicated for breaking ties with the EP and the Greek State Church - otherwise they too would have fallen under this canonical condemnation.
c) While often reasons of practicality and accuracy are cited as being the "real" reason for the New Calendar, this is simply not the case. The same
Patriarchal Synodical of the EP (1920) states quite explicitely that the purpose of the "new calendar" (really just the old Papal calendar) was to facilitate
ecumenism.
d) Ecumenism is a
heresy (illicit innovation.) Many argue against this, stating that there is "good ecumenism" and "bad ecumenism." Allegedly, the "good ecumenism" involves witnessing to the Faith in front of non-Orthodox. However, this is nothing new - if that is what ecumenism
really was about, there would have been no need for giving a new name to this - rather this is simply the age old apostolic mission of the Church.
Sadly, this is not what the actual
ecumenical movement is about - and it's not what the ecumenism desired by the EP's
Synodical of 1920 is about either! Please read the text - it is imbued with a false ecclessiology (ecclessiology - theology of the Church, doctrine about the Church, and what She is).
The
ecclessiological heresy of ecumenism, has at it's root, the subtle, easy to miss assumption, that there is a need to restore a
lost unity. This is heretical, for the Church has not lost it's unity, ever, for She is the Body of Christ - rather, individuals (sadly, lots of individuals) have left Her.
Even the name for this enterprise is heretical - in Orthodox usage, "ecumenical" denotes something
within the Church, pertaining to the universality/catholicity of the Church. Thus, why the great councils are descibed as
Ecumenical Councils. Thus, to speak of an "ecumenical movement" means one is trying to recover the supposedly lost/damaged "unity" of the Body of Christ. This is blasphemy.
The Patriarchal Synodical, which innaugerated the new calendar innovation, in addressing non-Orthodox bodies, addresses them as the "Churches of Christ everywhere" and that "fellowship between them is not excluded by the doctrinal differences which exist between them" (!!). This would be news to the Fathers, who accepted the Apostolic Canons (which are universally accepted in Orthodoxy; you'll find them in any edition of the Rudder, the compendium of Orthodox canons), which tell us...
Canon XLV - Let a bishop, presbyter, or deacon, who has only prayed with heretics, be excommunicated: but if he has permitted them to perform any clerical office, let him be deposed.
or
Canon LXIV - If any clergyman or layman shall enter into a synagogue of Jews or heretics to pray, let the former be deposed and let the latter be excommunicated.
Yet, unashamedly, the EP and those who are in communion with him (including the various heirarchs, archpastors, and patriarchs of the "official", in realit psuedo, Orthodox world) make quite a regular practice of not only praying with heretics in their houses of worship, but even performing liturgical acts with them, and carrying on with papalist clergy as if they were in fact genuine Priests.
This basic confusion about the Church's distinct identity, has obviously resulted in other related errors (for example, the common misunderstanding that there are genuine mysteries outside of the Orthodox Church - a total misunderstanding of the Church's economy in receiving converts, which is why so many genuine Orthodox Bishops now refuse to do this, but will receive converts only by exactitude, that is, baptizing them in water, so as to avoid any confusion.)
To see an official, online edition of the Patriarchal Synodical of 1920, go here.
http://www.patriarchate.org/encyclicals/patriarchal_encyclicals/Encyclical_1920
For written/pictoral examples of "Orthodox" heirarchs violating the canons, and undermining dogmatic principles with heretics, see the following...
http://www.russianorthodoxautonomouschurchinamerica.com/kissofjudas/kiss_of_judascontents%20page%20new1.htm
Seraphim