The Gregorian reform of the calendar meant that ten days were added to the calendar (October 5th became Octover 15th) and leap years were not counted on centenials not divisible by 400. Hence the years 1700, 1800, and 1900 turned from leap years into common years, while 1600 and 2000 remained leap years.
According to wikipedia this means that by the 20the century there was a difference of 13 days between the previous Julian, and Gregorian calendars (10 plus 3 =13. subtract 13 to get the Julian equivalent).
But it seems to me that that the Julian calendar would have had three extra leap years in 1700,1800,1900 meaning that it would have caught up three days with the Gregorian calendar which did not have them. Hence I would have thought the difference would be 10-3 = 7 days. Where am I going wrong?
I am wrong because even this online calendar date converter subtracts 13.