Because the old Julian calendar system--promulgated in the first century BC by Julius Caesar--made each year too long by several minutes, the calendar was displaced by three full days every 400 years. Thus, by the middle of the sixteenth century it was out of sequence with the equinoxes by a full ten days and with the original dates by fourteen days.
The energetic and reform-minded Pope Gregory XIII was not one to let such a problem continue indefinitely. He determined to correct the calendar and so on February 24, 1582, acting on the recommendations of a special council, issued a bull requiring all Catholic countries to follow October 4 with October 15 that year. To many of the common folk, it seemed as if eleven days had actually been stolen from them and panic ensued in several towns and villages throughout central and southern Europe.
The new Gregorian calendar was only reluctantly adopted by countries around the world during the next three centuries, but eventually, it became the standard measurement for the passing of time.