# Why are there 10 days difference in Hijri and Gregorian calendar?

I came upon this book that has written dates in Hijri and Gregorian calendars side by side. However, when I converted the dates on online converters there was a difference of 10 days. why is that ? in book : 6th Sept 1657 = 7th Zil.H 1067 conversion from online converters: 16th Sept, 1657 = 7th Zil.H 1067?

• When and where was the book written? Many Protestant states didn't switch over to the Gregorian calendar until the 18th Century. So it's possible the dates are using the Julian calendar. Jan 12 at 10:29
• @MCW I think the problem is that his sources map the same Hijri date to two different "Christian" dates, although there should be a one-to-one mapping.
– Jan
Jan 12 at 14:12
• @Steve Bird another possibility is that the books follow the calendar of the place they write about. E.g. it is quite conmon to state that the October Revolution took place on October 25th.
– Jan
Jan 12 at 14:16
• As mentioned, it's probably something to do with Old Style and New Style dates but without more information it's hard to say exactly. Can you explain in more detail what the book says and what exactly you calculated. Jan 12 at 15:45
• @Jan pre 1752 - in India there would be 2 "Christian" dates the julian for English based and gregorian for French and Portugese based so there is no one-to-one mapping - also we don't know what oneline converter the OP used Jan 12 at 19:18