In 45BC Julius Caesar introduced a new calendar with 365 days in a "normal" year and 366 in every fourth year or "leap year". This calendar became known as the Julian calendar. However, the Julian calendar was 11 minutes and 14 seconds longer than the solar year, a discrepancy which accumulated over time until by 1582 the vernal equinox occurred 10 days early and church holidays did not occur in the appropriate seasons. In order to make the vernal equinox occur on or around 21st March, Pope Gregory XIII decreed that ten days should be dropped from the calendar. In order to prevent the discrepancy from reoccurring the Pope introduced a new calendar in which leap years still occur every fourth year except where it is a century year (i.e. divisible by 100), in which case it is a normal year. However if the year is also divisible by 400 then it becomes a leap year. Thus 1600 was a leap year, but 1700 and 1800 were not. This new calendar was known as the Gregorian calendar.
The Gregorian calendar was slowly introduced across Europe, but in England the public were alarmed at the idea of dropping 10 days from the calendar - they believed their life-span was pre-determined, and that removing 10 days from the year would result in everyone dying 10 days early! As a result the Gregorian calendar was not implemented in Britain until 1752, by which time it had become necessary to drop 11 days from the year. More significant as far as genealogists are concerned is that when the Gregorian calendar was introduced in Britain the first day of the New Year was also adjusted. In the Julian calendar New Year's Day had been Annunciation Day on 25th March, but in Britain it was changed to 1st January.
This adjustment in the first day of the New Year causes problems for genealogists. In the Julian calendar 31st December 1700 was followed by 1st January 1700, which means that an individual can appear to die before being born! For example, someone could be baptised on 1st December 1700 and buried on 1st January 1700. However, if the 1752 adjustment of New Year's Day is back-dated, the burial date becomes 1st January 1701, which then makes sense. In order to clarify things some genealogist use what are known as "double dates", in which both the Julian and Gregorian year is expressed. In the example given above the double date for the individual's burial would be 1st January 1700/1701, indicating that while the original parish record might show the burial to have occurred in 1700, it in effect actually occurred in 1701.
On my website I have decided not to use double dates and to simply use the date as stated in the original parish record, where possible. This can result in some unusual events, but I feel that using double dates would make my website look untidy. Anyone reading these pages should simply add one year to any pre-1752 date between 1st January and 24th March in order to determine the modern equivalent of that date.