Calendars Across Time
Pick one moment and see it in every calendar at once โ the one on your wall, the ancient Roman one, the Moon-based Islamic one, the Maya odometer, and the running day-counts astronomers and computers use. Drag the slider and watch them all change.
Because nature handed us three clocks that don't divide evenly. A day is one spin of the Earth. A month is roughly one orbit of the Moon (~29.5 days). A year is one trip around the Sun โ and that trip takes about 365.2422 days, not a tidy 365. Every calendar is a different clever hack to paper over those leftover fractions.
Solar calendars (Gregorian) track the Sun and the seasons, adding leap days to soak up that extra quarter-day. Lunar calendars (Islamic) follow the Moon and let the seasons drift. Lunisolar calendars (Hebrew, Chinese) try to honor both by adding a whole leap month some years.
The 10 days that never happened. By 1582 the old Julian calendar had drifted 10 days off the seasons, so Pope Gregory XIII simply deleted them: in Catholic countries, the day after Thursday October 4 was Friday October 15, 1582. Set the tool to that week and watch Gregorian and Julian disagree by exactly 10 days.
The Maya "apocalypse" was an odometer rollover. On December 21, 2012 the Maya Long Count ticked over to 13.0.0.0.0 โ the end of a great cycle. Nothing ended; it's exactly like a car's odometer rolling from 99,999 to 100,000. Try it in the tool.
Astronomers and computers gave up on months entirely. To compare events across thousands of years and dozens of calendars, they just count days (the Julian Day Number) or seconds (Unix time) as one ever-growing number. Simple, unambiguous, and never needs a leap-month.
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