at the height of the Church’s calendar problem, in the second half of the 16th century, the eastern Church and the western Church were an incredible ten days out of sync with one another. This was only reconciled in 1582 when Pope Gregory XIII implemented what has become known as the Gregorian calendar reform.
This doesn't sound right at all. Why would the church calendars be out of sync at the time? In fact, it was the Gregorian reform that put them out of sync. They are still out of sync and the gap is growing.
Bonus history trivia question: What month did the Russian October Revolution happen?