Convert Years to Days

“How many days in a year?” doesn’t have a single answer.
365: a common (non-leap) year – what calendar years usually have.
366: a leap year – what 2024, 2028, 2032 have.
365.25: the Julian average – used in astronomy (the “Julian year”) and was the old Western calendar before 1582. Slightly too long by ~11 minutes per year.
365.2425: the Gregorian average, accounting for the every-100-but-not-400 leap rule. This is the long-term average of the modern calendar, correct over centuries but never exactly right for any single year.

For specific years (calendar planning): 365 or 366. For long durations or averages (lifespan, project planning): 365.2425. For astronomy or old documents: 365.25.

Because no single number fits every question. 365.2425 works for “how many days in a 50-year career” but is wrong for “how many days in 2025” (a non-leap year, so 365). Making the choice explicit keeps the answer honest in both cases.

Those are the tropical year (365.24219 days, time between vernal equinoxes – the natural year) and another rounding. The Gregorian calendar uses 365.2425 because it’s a rational approximation easy to express in leap-day rules (97 leap years per 400 years). For most practical use, the Gregorian value is close enough to tropical.

It doesn’t. DST changes wall-clock time, not the abstract 24-hour “day” used in this math. A “day” here is exactly 86,400 seconds.

Gregorian rule: a year is leap if divisible by 4, EXCEPT century years (divisible by 100), EXCEPT century years also divisible by 400. So 2000 was leap, 1900 wasn’t, 2100 won’t be. This gives 97 leap years per 400-year cycle, hence 365 + 97/400 = 365.2425.

Because Gregorian average is 365.2425, not 365. For N years, the average has N × 0.2425 extra days. If you wanted exactly N × 365 days, choose the “Non-leap” convention.

Both are rejected with explicit error messages rather than silently producing wrong output. The error names the offending value, so you can fix the input instead of hunting for the problem.

No. Everything runs in your browser with JavaScript – nothing is sent to a server, logged, or stored, and the tool keeps working offline once the page has loaded.

Same math, inverse direction. Swap this tool, or use the dedicated days-to-years tool (if it exists in the suite) – both should produce identical results with matching conventions.