Dating the calendar
Muslim calendar, also called Hijrī calendar or Islamic calendar, dating system used in the Muslim world for religious purposes.(Most countries now use the Gregorian calendar for civil purposes.) It is based on a year of 12 months, each month beginning approximately at the time of the new moon.By that time, the discrepancy between a solar year and the Julian Calendar had grown by an additional day, so that the calendar used in England and its colonies was 11 days out-of-sync with the Gregorian Calendar in use in most other parts of Europe.England's calendar change included three major components.
The beginning of the legal new year was moved from March 25 to January 1.
To avoid misinterpretation, both the "Old Style" and "New Style" year was often used in English and colonial records for dates falling between the new New Year (January 1) and old New Year (March 25), a system known as "double dating." Such dates are usually identified by a slash mark [/] breaking the "Old Style" and "New Style" year, for example, March 19, 1631/2.