The Eugenian calendar

The problem with the Julian calendar was recognized from the very beginning, well before Andalusada's POD:

While Hipparchus and presumably Sosigenes were aware of the discrepancy, although not of its correct value, it was evidently felt to be of little importance at the time of the Julian reform. However, it accumulated significantly over time: the Julian calendar gained a day about every 134 years.

Why is this remotely relevant? Because calendrical reform affects everything. The decisive rupture between Russia and the rest of the Orthodox world starts with a calendrical reform, and the Vechist Wars were directly tied into the politics of the liturgical year. (It's still being fought over as of the present day.)

Calendar reforms before Evgeny the Great

Evgeny the Great is the tsar who gets the calendar named after him, but he's not the first person to propose it.

• By the time he puts on his Peter the Great panties, the Julian calendar's problems were already being discussed at length; the last antipope [who?], in fact, had reformed the Julian calendar during his reign. (Having been declared an antipope, it was disestablished after his death.)
• The Church of Hungary, at this point no longer in communion with anybody, was also going to address this. Several models were proposed; one of them [which?] was, in the end, adopted by Evgeny I for himself.

Armed with what would become the Eugenian calendar, Evgeny I now faced the single biggest problem of the whole plan: getting it approved for use as a liturgical calendar by the Orthodox communion.

The ecumenical rejection of the Eugenian reforms

The Orthodox world had at least three very good reasons to reject it:

• "We, the Pentarchy, use the Julian calendar, and we're older than you, thus more Orthodox. Unless we adopt it too, you'll anathematize yourself by no longer celebrating the great feasts on the same day as all other Christians."
• "The Latins who you want to keep up with aren't Christians and haven't been since they unilaterally broke ties with us in 1055. (Also, they did it first.) You want to yoke believer and unbeliever equally, for no reason other than national pride."
• "Our Mohammedan overlords use the Julian calendar as a de facto civil calendar themselves, because at least it tries to correspond to the seasons. You want us to adopt your calendar, take it up with them."

At that point, Evgeny I was screwed, and because he did promulgate the Eugenian calendar, the Pentarchy solemnly anathematized Tver, recognizing only the remnant Metropolitanate of Kiev as truly Orthodox. Great Russia's break with the historic Orthodox churches was complete.

The consequences of the Eugenian calendar

No calendar reform ever goes over perfectly. The Eugenian one was particularly messy:

• Because "the Orthodox dark age" comes later (the fall of Constantinople is in 1476, after all), and the Raskol comes earlier, the Metropolitanate of Kiev (which does remain in the Orthodox fold) is reduced to impotence, which drives the parts in Poland-Ruthenia into the waiting arms of Rome.
• Evgeny I, pretty much as per IRL, faces at least one enormous revolt immediately after.

In a broader sense, the Eugenian calendar forces the distinction between the Levantine Orthodox (who stick with the Julian calendar) and the Russian Orthodox (who follow the Eugenian.)

The Eugenian calendar today

As of the present day, the Eugenian calendar remains the civil calendar of Russia (now Great Russia, thank you.) It's also the official calendar of Korea, and is gradually gaining support in Anatolia, where most of Great Russia's military satrapies are adopting it as well.

It remains the de facto liturgical calendar of the Russian Rite churches.

• Most of the Tsarist churches follow the Eugenian calendar exclusively.
• Most of the Russian autonomous churches still use it; this is the calendar followed by Koreans (at home and abroad) and the Ezoic diaspora, including the Meammosiran Cossacks.

Arguments about which calendar to adopt, and why, are the single biggest cause of dissension in the Orthodox world at the moment.