📅 24 February 1582
Pope Gregory XIII issued the bull Inter gravissimas. It reformed leap-year rules to bring the civil calendar closer to the solar year.
📅 24 February 1582
The reform used astronomical calculation and a revised leap-year cycle to limit the drift of the equinox through the calendar.
📅 24 February 1582
The calendar illustrates how numerical models become social institutions through political and religious authority.
📅 24 February 1582
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.
📅 15 October 1582
In several Catholic territories, Thursday 4 October was followed by Friday 15 October 1582. Ten numbered dates were omitted to realign calendar and season.
📅 15 October 1582
Century years became leap years only when divisible by 400, producing a mean year closer to the tropical year than the Julian rule.
📅 15 October 1582
Events near the reform may have Old Style and New Style dates. Databases must identify the calendar before comparing dates.
📅 15 October 1582
European empires later carried Gregorian dating into colonies, while many communities retained religious and Indigenous calendars alongside it.
