Convert years to days

The formula

days = years × 365.2425

years to days — quick reference

Which year, and it is not a pedantic question

A common year is 365 days and a leap year 366, and no calendar year is anything else. But an average year is not a whole number: the Gregorian mean is 365.2425 days, because a year divisible by four is a leap year unless it is a century not divisible by 400 — which is why 1900 was not a leap year and 2000 was. Astronomers use the Julian year, exactly 365.25 days, and that is the year inside the definition of a light-year.

The mean tropical year, the actual interval between March equinoxes, is 365.24219 days, and the Gregorian rule was designed to track it. Over ten years the choice between 365 and 365.2425 is two and a half days; over a human lifetime it is nearly three weeks. This page uses 365.2425, the calendar average.

Finance invented a year that does not exist

Bond markets and loan schedules routinely count a year as 360 days of twelve 30-day months, the convention written as 30/360. It survives from an era of hand calculation, and it means a month of interest is the same whether it is February or July. Alongside it live actual/365, actual/360 and actual/actual, and they disagree.

The gap is not decorative. On a million-unit balance at five per cent, actual/360 collects about 1.4 per cent more interest over a year than actual/365 — the same rate, a different definition of a year. If a figure came from a loan schedule or a coupon calculation, convert with the convention the contract names, not with the calendar.

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