1582, why are there days less than 10?

1582 lost 10 days because of the imperfection of the previous calendar.

Although the calendar at that time was very accurate, the setting of leap year was different from that in period of revolution, which was about every 128 years 1 day. In this way, by the second half of16th century, the deviation accumulated to 10 days.

1582, Pope Gregory XIII revised the calendar again. The leap year is changed from every 400 years 100 to 97 every 400 years, that is, a leap year that can be divisible by 4. Every century year must be divisible by 400 to be considered a leap year, so that there will be a deviation of 1 day in 3300 years. This is the common calendar in the world today-Gregorian calendar, or Gregorian calendar for short.