Chronology Lab

Calendar history

Trace the major reforms that shaped how we mark days and years.

Key reforms

Milestones

From the Julian calendar to UTC, these milestones kept our civil count aligned with astronomy and each other.

  • 46 BCE – Julian calendar introduced leap years to keep pace with the Sun.
  • 1582 – Gregory XIII skipped 10 days and refined leap-year rules (1500 and select centuries now stay 365 days).
  • 1752 – Britain & the American colonies adopted Gregorian; calendars jumped ahead 11 days overnight.
  • 1972 – Coordinated Universal Time introduced leap seconds for atomic stability.

Why the tweaks mattered

Impact

Each calibration prevented seasonal drift. Administrators, astronomers, and navigators relied on predictable leap rules, which still influence compliance calendars and release planning today.