The Gregorian calendar was introduced in 1582 to correct drifting dates in the older Julian calendar.

The longer version

By the sixteenth century the equinox had slipped about ten days from where the older calendar placed it, because the Julian rule added a day slightly too often. Reformers computed new leap year rules and deleted the accumulated days in one step. Ten dates were dropped from October that year in adopting countries.

The correction was designed around the date of Easter, which is tied to the spring equinox. Astronomers consulted by the Church supplied tables, and the change was imposed by papal decree. Because it came from Rome, several states ignored it for a long time afterwards. The new rules kept only century years divisible by four hundred.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.