The Gregorian calendar was first adopted in Italy, Poland, Portugal and Spain in 1582.

The Gregorian reform consisted of the following changes:

• 10 days were dropped in October 1582.
• New rules were set to determine the date of Easter.
• The rule for calculating Leap Years was changed to include that a year is a Leap Year if:
  1. The year is evenly divisible by 4;
  2. If the year can be evenly divided by 100, it is NOT a leap year, unless;
  3. The year is also evenly divisible by 400. Then it is a leap year.

In September 1752, Great Britain switched from the Julian Calendar to the Gregorian Calendar.

In order to achieve the change, 11 days were "omitted" from the calendar - i.e. the day after 2nd September, 1752 became the 14th September, 1752.

However, many countries used the Julian Calendar much longer.

Turkey was the last country to officially switch to the new system on 1st January, 1927.