After centuries a small inaccuracy of the eleven and a half minutes per year ads up and by the sixteenth century the little but continuing minutes had put the Julian calendar behind the solar one 10 days. Under Cesar's rule some one would have been thrown to the lions for such an inadequacy.
An advancement of 10 days was no longer tolerant so pope Charles IX and new correction device to curb further error was Introduced, making century years of 1700 or 1800 wouldn't be counted as leap years unless they are divisible by 400. The new Gregorian calendar system is very precise and still used in the United States and systematically only errors 26 seconds per solar year a deficient of one day every 3323 years.
The wise and practical change by pope Charles was uninformed by many and ignored by some and protestant from those that wanted no change. And change for some was as late in 1918, Russia converted after there revolutionary war , Greece finally change in 1923.In contrast still many orthodox churches follow the Julian calendar to the day. Mean while the Julian calendar has a deficient of 13 days to date by the