There is a term in chronology called an era
. It doesn't matter what year it's now it has to have it's sequence number, i.e. it has to be counted from some starting date.
Actually, the term era is an abbreviation of latin phrase "ab exordio regni Augusti" i.e. "since the reigning of Augustus" (aera - era)
Take notice that era can be real - when it comes from a real event, like since the reigning or unreal - when it comes from a mythical event, like the world creation.
We know one of such eras - Christian era or Anno Domini
It was created by the Roman monk Dionysius Exiguus in circa VI AD. Then the so called "Diocletian era" i.e. the numbering of years was since the reigning of Roman emperor Diocletian.
Dionysius somehow calculated that the date of Christ's birth was 284 years before the Diocletian era. Dionysius era was adopted in all Christian Europe.
It was different for Russia. As Christianity came to us from Byzantium, it came with Byzantium chronology since the creation of the world
as well. This system was used in Russian until 1700 when by Peter I decreed to convert Russia into Christian era.
According to Byzantine chronology, there were 5508 years from the creation of the world till the birth of Christ. The year in this system is based on a Julius calendar.
It seems that the conversion is pretty simple, but in Ancient Russia until the end of XVII century the year started from March or September. I.e. until the Peters I decree there were two calendar styles - march(the year started from the 1st of March) and September(the year started from the 1st of September).
Different styles change the method of calculation as for the march style the year is lagging behind the Christian new year and for the September style it's ahead of Christian new year by 4 months. Here is an example.
Let's use March of 7100 in march style. It matches March 1592 AD.
If we use February 7100 in march style, i.e. the end of the year, it will match February of 1593 AD.
Now let's review September 7100 in "September style". It matches September 1591 AD, but February 7100 matches February 1592 in "September style".
By the way, it's not stated which style is used for the dates of events in chronicles. Although, there are numerous logical methods helping to find the style used in chronicles. It's also known that since the end of XV century the September style was used more then march style. There was also two modification of march style - Ultra-march and circa-march.
So this calculator converts dates of our era into Old Russian (Byzantine) and can be used for mere fun. The backconversion is used of dating of chronicles and more complex as it requires context analysis of style used in the chronicle.
The last thing is about months - as the Julan calendar was a basis for an old Russian calendar in early sources there are latinized names of the months, e.g. Junius, Julius, Augustus etc.