A calendar from the year 813 discovered in Florence that corrects the gap between the Julian calendar and the solar year

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A calendar created in 813 has been rediscovered in Florence, where the discrepancy between the Julian calendar and the solar year is already corrected. This discovery predates the full recognition of this phenomenon by more than seven centuries, which was only formalized with the Gregorian Reform of 1582. The discovery is the result of a study conducted by Francesco Vizza, an emeritus associate researcher at the National Research Council, former director of the Institute of Chemistry of Organometallic Compounds (CNR-ICCOM), and a scholar of the history of the Gregorian calendar, with the support of Giuseppe Giari, archivist of the Opera di Santa Maria del Fiore. The manuscript, preserved in the Laurentian Library, reveals surprisingly advanced astronomical expertise. Its compilers recognized that the chronological system then in use did not precisely follow the true motion of the Sun, and that the dates of celestial phenomena were already approximately three days ahead of the official calendar dates.