Time Converter

This converter uses 365.25 days as a practical average, but that number is one of several genuinely different definitions of a year, each built for a different job.

Tropical, sidereal and Julian years all disagree

A tropical year, about 365.2422 days, is the time for the seasons to repeat and is what calendars are built to track. A sidereal year, about 365.2564 days, is the time for Earth to return to the same position relative to the distant stars, slightly longer because the Earth's axis slowly precesses. A Julian year, defined as exactly 365.25 days, is an astronomical convention with no attempt to match either real cycle, used mainly because it gives light years and other astronomical distances a fixed, unambiguous basis. None of the three is wrong. They simply answer different questions.

Timekeeping also corrects for seconds, not just years

Earth's rotation is not perfectly steady. Tidal friction from the Moon slows it very gradually, and shorter term variations move it around unpredictably. Because atomic clocks keep time far more precisely than the planet spins, timekeeping authorities have periodically inserted a leap second into official clocks to keep clock time aligned with the actual day. The practice caused enough software problems, from time jumps to sorting bugs, that international standards bodies have voted to phase leap seconds out by 2035 rather than keep patching around them.

Finance uses its own invented calendar

Bond and loan interest is often calculated on day count conventions that don't match the real calendar at all. The common 30/360 convention treats every month as exactly 30 days and every year as exactly 360 days for the sole purpose of computing interest, regardless of how many days actually passed. It isn't an error or an approximation, it's a deliberately simplified unit built so that interest calculations come out the same regardless of which months a loan happens to span.