The Difference Between Julian and Gregorian 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.