Using the Julian and Gregorian Calendars
This is a very complex topic and this article deliberately tries to simplify the topic by missing out a lot of the details.
The UK adopted the Gregorian calendar system that we still use today in September 1752. This was around 170 years after most other European countries. Before 1752, the Julian Calendar was in operation. The main reason for doing this was that the Julian Calendar wasn't accurate enough in dealing with the true length of the solar year. Every 131 years the date would be out of sync by a day. To deal with this although both calendars had leap years every 4 years, the Gregorian Calendar stated that a year was a leap year if divisible by 4 but not if divisible by 100 unless also divisible by 400. i.e. the years 1600 and 2000 were leap years but 1700, 1800 and 1900 weren't.
At the same time that the Gregorian Calendar was introduced the start of the year in the UK was considered to be 1st January. Prior to this it was usual to consider 25th March to be the start of the year. Confusingly some records (including parish records) started using January 1st as the start of the year long before 1752 while other records continued to use the 25th March.
How does this affect Genealogists?
When your research reaches the 18th century you will need to be particularly careful when recording dates to work out whether the start of the year is assumed to be 1st January or 25th March. If for example you find a marriage on March 1st 1740 and then the baptism of the first child on April 5th 1741 and you record the information it might appear that the baby was baptised 13 months after the marriage. However, assuming the church used the Julian calendar at that time then the baby was actually baptised a month after the marriage! It is a good idea to look at the other entries in the register around that date to see when the year changes so that you can work out which system was being used. Be extra careful when using pre-1752 dates from the IGI. The IGI has no facility for recording dates in the correct dual format and all such dates should be checked in the original register to avoid confusion.
Recording Julian Calendar dates in Family Historian
If you are recording a date in Family Historian it is recorded as a standard Gregorian calendar date. However, when recording Julian calendar dates which take place before 25th March it is usual to put a double year to show the year as recorded in the Julian Calendar and what it would actually be if written in the Gregorian Calendar.
Hence a marriage on 1st March 1740 would be recorded in Family Historian as:
1 Mar 1740/41
because the Julian date 1st March 1740 would be considered to be 1st March 1741 if the Gregorian Calendar had been used instead with the new year beginning on 1st January.
The baptism on April 5th 1741 would still be written as:
5 Apr 1741
By recording the date of marriage as 1 Mar 1740/41 and the baptism as 5 Apr 1741 it is much easier to spot that these events were just a month apart. If you opted to just record the date as 5 Apr 1741 it would be difficult to know whether you mean 1740/41 or 1741/42.
The GEDCOM standard, and therefore Family Historian, also allows dates to be explicitly entered as Gregorian or Julian calendar entries, although this method is not commonly used. To use this method the Julian calendar date of 1 March 1740 would be recorded as:
[J] 1 Mar 1740 (The J being in square brackets)
Family Historian Preferences for start of Gregorian Calendar
For the purposes of calculating days-of-the-week and validating dates (between 1582 and 1752), Family Historian allows you to choose either the 1582 changeover date, or the 1752 changeover date (see Preferences). If your dates are wholly or mainly for Great Britain, Ireland and/or America, you may wish to use 1752. However, this is not a key decision. You can switch back and forward between 1752 and 1582 whenever you wish to. Doing so will not change the data in your file in any way. Nor does it matter if your file contains dates which would not be valid by one reckoning or the other (or even both!). Family Historian allows you to store invalid dates in any case. Apart from affecting the way that days of the week are calculated, the only other practical effect of changing the ‘changeover date’ setting is that it will affect which warnings you may get when you enter dates. However, you can always ignore these warnings and keep the date anyway. (From the Family Historian 3 on-line help)