For a well-researched account of the development of the modern calendar, see Duncan
The Julian Calendar
The Julian Calendar was used in Britain from the time of the Roman invasion until 1752. The Roman year exceeded the length of the mean solar year by just over 11 minutes, which meant that, after several centuries, the determination of Easter became difficult. By the 16th century, it was rapidly slipping into early summer.
The rule for leap years in the Julian Calendar was that the year must be divisible by 4. The extra day was added before February 25th.
The new year was generally taken in Britain to begin on Lady Day (March 25th), which led to the use of dual dates during the period leading up to the adoption of the new calendar. For example, you will find entries in parish records with dates such as 20th February 1719/20.
The Gregorian Calendar
In 1582, Pope Gregory issued a papal bull establishing calendar reform. The Gregorian Calendar was finally adopted in Britain in 1752.
The changes were as follows:
The rule for leap years was changed - the new rule states that a year is a leap year if:
- it is divisible by 4 and
- if it is divisible by 100, then it is also divisible by 400
So 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 are not.
The first day of the year is January 1st.
The position of the extra day in a leap year would be the day following February 28th.
To bring the Spring Equinox back to 21st March, eleven days had to be added to the calendar to compensate for the drift caused by the inaccuracy of the Julian Calendar. This was achieved by "losing" the 4th to the 13th of September, so that 3rd September 1752 was immediately followed by 14th September.
This caused an outcry from the public, who thought that eleven days had been "stolen" from their lives and demanded them back!
It also accounts for the strange date for the beginning of the financial year. When the 11 days were removed from the calendar in September, what would have been 25th March became 6th April. The financial year thus remained the same despite the changes to the normal calendar.
The Julian Calendar started with the year 1, not 0, as the Romans had no concept of zero and no symbol to represent it. This means that the decades end in tens (10, 20, etc.), the centuries end in hundreds (100, 200, etc.) and the millennia end in thousands (1000, 2000, etc.).
The end of the 2nd millennium was, therefore, 31st December 2000, with the new millennium beginning on 1st January 2001.