A calendar from 1079 keeps better time than ours
Omar Khayyam, better known today for his poetry, directed the commission of astronomers that produced the Persian Jalali solar calendar, inaugurated in 1079. Its scheme drifts from the true solar year by only about a day in several thousand years — a smaller error than the Gregorian calendar Europe adopted some 500 years later, in 1582.
How we know
Omar Khayyam (b. 18 May 1048, Nishapur – d. 4 Dec 1131); Jalali/Jalālī calendar inaugurated 15 March 1079 under Sultan Malik-Shah (Khayyam led the ~8-astronomer panel); accuracy ~1 day in ~5,000 years vs Gregorian ~1 day in ~3,330 years; Gregorian reform 1582 (1582−1079 = 503 years). Sources: Wikipedia (Omar Khayyam; Jalali calendar), MacTutor.
One of many fact-checked wonders across eight traditions at The Spring of Wisdom.