Gregorian VS. Julian Calendars: The Gregorian Calendar was first introduced by Pope Gregory XIII – which is how the calendar got its name. This calendar has been implemented by several countries because the Julian calendar assumes a full year is 365.25 days whereas it is actually 11 minutes less. So, the Julian calendar many countries felt wasn’t a true year so they made the change. The Gregorian calendar was able to make up for this 11 minute difference by not making years divisible by 100 to be a leap year. This means that the year 2,100, for example wouldn’t be a leap year whereas in the Julian calendar format – it would be. So, the difference in the two calendar formats do not seem to be very profound. However, historians, scribes, statisticians, and weather experts for example, are very well aware of any changes in calendars and dates. Most countries today use the Gregorian calendar. The time periods vary as to when countries migrated from the Julian to Gregorian calendar. Spain, Italy, and France for example switched over in 1582. Great Britain didn’t switch over to the Gregorian until 1752. Other countries didn’t switch over until more recent times – Greece for example was using the Julian all the way up until 1922. Many people wonder why Russia didn’t switch over to the Gregorian calendar when the rest of Europe did. The belief is that Russia did this because the calendar was introduced by a Roman Catholic pope. Russia ended up switching over to the Gregorian in 1918.
Old Style (O.S.) and New Style (N.S.): Old Style and New Style are terms sometimes used with dates to indicate that the calendar convention used at the time described is different from that in use at the time the document was being written. There were two calendar changes in Great Britain and its colonies, which may sometimes complicate matters: the first change was to change the start of the year from Lady Day (25 March) to 1 January; the second was to discard the Julian Calendar in favor of the Gregorian Calendar. Closely related is the custom of dual dating, where writers gave two consecutive years to reflect differences in the starting date of the year, or to include both the Julian and Gregorian dates.
Other dates can be used are: Date of Baptism or an approximate date.