The Gregorian calendar was first introduced in 1582, to compensate for the fact that the earth does not go around the sun in 365.25 days, but in 365.2425 days. The years were passing slightly faster than the calendar indicated. In 1582 the calendar system was 10 days behind. So after October 4th, came October 15th and a new and improved leap year scheme was introduced. England and its colonies (including what is now the eastern United States) only made the correction in 1752.
Check out the related Date Difference Calculator to calculate the time difference between two dates.
Written by Otto de Voogd