Question
When did the civil calendar that we use today [in the western world] begin?
--Curious in Canton
Answer
From the Hartnett Jesuit Newsletter for October 1, 1998 and Gerard Delaney:
October 5, 1582: The Gregorian calendar, which had been worked out by Christopher Clavius, SJ, went into effect. It allowed for the one-time suppression of the days between October 5 and 15 in order to bring the calendar in line with astronomical facts. Countries which did not like the Pope liked his calendar even less. It actually took until the 20th century until all countries adopted it as their civil calendar.
The Gregorian calendar was developed under Pope Gregory XIII (pope 1572-1585) and promulgated in 1582. So the year 1600 was a leap year in most of the Western world, except in England and her American colonies. George II finally bowed to the logic of the situation (especially when the spring equinox occurred in April), in 1752, by which time the Julian calendar was 11 days off.