The older Julian system miscalculated the solar year by about 11 minutes. Over 1,600 years, this caused the calendar to drift by roughly 10 days. This drift concerned church leaders because Easter was drifting further away from the spring equinox.

To fix this, Pope Gregory XIII issued the papal bull Inter gravissimas in 1582, based on the design of astronomer Aloysius Lilius. The reform aimed to accomplish two goals:

1. Recalibrate the calendar so the spring equinox fell on March 21.

2. Adjust the leap year rule so the calendar would stay synchronized with the seasons.

The 1582 Shift to realign the calendar, 10 days were skipped in October 1582. In Catholic nations, October 4 was immediately followed by October 15.

Global Adoption Because it was introduced by a pope, non-Catholic countries initially rejected the new calendar, causing a temporary logistical headache in Europe where a short border crossing could mean jumping 10 days into the future or past