A papal decree standardizes a mathematical system

Libraries and institutions suffered catastrophic loss, although the familiar story that all books were thrown into the Tigris cannot be treated as a precise inventory.

Knowledge survived through manuscripts and scholars in Maragha, Damascus, Cairo, Central Asia and other centers, preventing a simple rise-and-collapse narrative.

The destruction of Abbasid institutions redirected scholars and resources, reshaping rather than ending mathematics in the Islamic world.

Pope Gregory XIII issued the bull Inter gravissimas. It reformed leap-year rules to bring the civil calendar closer to the solar year.

The reform used astronomical calculation and a revised leap-year cycle to limit the drift of the equinox through the calendar.

Catholic states adopted the reform first, while other societies accepted, adapted or resisted it at different times; calendar history was not a single global switch.

In several Catholic territories, Thursday 4 October was followed by Friday 15 October 1582. Ten numbered dates were omitted to realign calendar and season.

Century years became leap years only when divisible by 400, producing a mean year closer to the tropical year than the Julian rule.

Events near the reform may have Old Style and New Style dates. Databases must identify the calendar before comparing dates.

European empires later carried Gregorian dating into colonies, while many communities retained religious and Indigenous calendars alongside it.