Meanwhile, in Persia—

With only a jug of wine, a loaf of bread and thousands of calculations, Omar Khayyám works out a new calendar

ad1075. In Persia, using the Hindu-Arabic decimal system, Omar Khayyam introduced a new calendar. The length of the year was measured as 365.24219858156 days and is very accurate. It didn’t catch on in the West.

https://www.britannica.com/biography/Omar-Khayyam-Persian-poet-and-astronomer

Omar Khayyam

https://www.timeanddate.com/calendar/persian-calendar.html