The leap-year rule is the key difference
The Julian calendar treats every fourth year as a leap year. That rule is simple, but it makes the average year slightly too long compared with the solar year. The Gregorian calendar keeps most fourth-year leap years, but century years are leap years only when divisible by 400.
That adjustment is why 1900 was a leap year in the Julian calendar but not in the Gregorian calendar, while 2000 was a leap year in both. Over centuries, these small rule differences create a visible date gap.
Calendar reform was not simultaneous
Many Catholic regions adopted the Gregorian reform in 1582, but other countries changed later. Some civil records, church records, and local documents continued with the Julian calendar long after neighboring regions had moved to the Gregorian calendar.
This means historical conversion is not only a math problem. You also need the place, institution, and record type when a date falls near a local reform period.
The same written date can therefore be correct in one archive and misleading in another. Use the Julian Calendar Converter for the arithmetic date equivalent, then compare the source context with the checklist in Avoiding Mistakes in Historical Date Conversion.
How to read converted dates
A converter can tell you the corresponding calendar date under a stated rule set. It cannot always tell you which calendar a historical writer intended unless the source context is known.
For scholarly or archival work, keep both forms in your notes when useful: the date as written in the original source and the normalized Gregorian equivalent used for modern comparison.