Omar Khayyam is the Persian mathematician and poet commonly credited with reforming the calendar that led to the Solar Hijri system.
In 1074–1075, Seljuk Sultan Malik-Shah commissioned Khayyam and a group of astronomers to reform Persian timekeeping. Working from an observatory in Isfahan, they made astronomical observations and designed the Jalali calendar, whose new year was tied closely to the vernal equinox.
The reform was inaugurated in 1079. Its solar structure and emphasis on the equinox helped establish the tradition behind the modern Iranian Solar Hijri calendar, which is also called the Persian calendar. Khayyam was not acting alone: other astronomers participated, so “credited with” is more accurate than saying he single-handedly invented the system.
A frequent mix-up is treating the medieval Jalali calendar and the modern Solar Hijri calendar as exactly identical. The modern civil calendar was standardized and modified centuries later, especially in Iran in the twentieth century, but Khayyam’s reform is its celebrated historical ancestor.