Time Unit Conversions Explained: Seconds, Days, and Why a Year Isn't Fixed

Time conversion looks like the easiest job on this site: 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day. Then someone asks how many days are in a year, and the honest answer turns into a question: which year? The ladder from seconds up to weeks is exact and unambiguous. Above weeks it stops being exact, and the gap between “exact” and “close enough” is where real errors get made, usually in payroll, contracts, and age calculations.

Where the exact ladder stops

The SI treats the second as the base unit and admits the minute, hour, and day as non-SI units accepted for use alongside it, with fixed values: 1 min = 60 s, 1 h = 3600 s, 1 d = 86 400 s (NIST SP 330, Section 4). The week isn’t in that table, but it inherits exactness from the day: 7 × 86 400 = 604 800 s, and nobody disputes that a week is seven days.

The month and the year are missing from that list, and not by accident. Neither one has a fixed number of seconds, so neither can be given an exact conversion factor.

Four different years, all correct

The calendar year most people mean is 365 days, except when it’s 366. The astronomical quantity the calendar is chasing is the tropical year, the cycle of the seasons, which the US Naval Observatory puts at approximately 365.2422 days (USNO, Leap Years). A calendar has to use whole days, so the Gregorian calendar approximates that fraction with a leap rule: years divisible by 4 are leap years, except centurial years not divisible by 400. That makes 1900 an ordinary year and 2000 a leap year, and it gives an average Gregorian year of 365.2425 days, within about half a minute of the tropical year (USNO, same page).

Astronomy uses a fourth figure. The Julian year of exactly 365.25 days is a convenience constant rather than a description of Earth’s orbit; it’s the year in “light-year.” NIST’s own conversion tables list the variants side by side: year (365 days) = 3.1536 × 10⁷ s, year (tropical) = 3.155 693 × 10⁷ s, year (sidereal) = 3.155 815 × 10⁷ s (NIST Guide to the SI, Appendix B.9). When the national metrology institute needs three rows for one word, the word needs qualifying.

Our converter uses the mean Gregorian year, 365.2425 days (31 556 952 s), because the question behind it is nearly always a calendar question rather than an orbital one. Pick the tropical year instead and you shift by about 26 seconds per year; pick the Julian year and you shift by about 11 minutes per year. Both are irrelevant for a project timeline and both matter if you’re propagating an orbit.

Why “a month” has no answer

A month is 28, 29, 30, or 31 days depending on which one you’re standing in. There is no averaging trick that makes this go away, because the thing people want from a month conversion is usually calendar-anchored, not duration-anchored. “Three months from March 31” has a defensible answer (June 30) that has nothing to do with multiplying by any number of days.

The average month on this site is 30.436875 days, one twelfth of the mean Gregorian year. It’s the right figure for spreading an annual cost across months or estimating a burn rate. It is the wrong figure for anything with a date attached. A 90-day payment term and a three-month payment term are different terms, and which one is longer depends on the month you start in. Financial contracts deal with this by declaring a day-count convention in the contract text rather than relying on a conversion; if you’re doing the arithmetic for a contract, find that clause before you pick a factor.

Reference table

Unit	Exact?	Seconds	Days
Second	Yes	1	0.0000115741
Minute	Yes	60	0.000694
Hour	Yes	3 600	0.0416667
Day	Yes	86 400	1
Week	Yes	604 800	7
Month (average)	No	2 629 746	30.436875
Common year	Fixed by definition	31 536 000	365
Leap year	Fixed by definition	31 622 400	366
Julian year (astronomy)	Fixed by definition	31 557 600	365.25
Mean Gregorian year	No	31 556 952	365.2425
Tropical year	No	≈31 556 930	≈365.2422

Rows marked “fixed by definition” are exact numbers, but they describe a chosen convention rather than a measured interval. A common year is exactly 365 days because that’s what a common year means.

Three conversions people get wrong

Hours worked to “days” in payroll. This is the most expensive one. If someone logs 37.5 hours and you divide by 24, you get 1.5625, a meaningless number that has nothing to do with their pay. Timesheets measure working days, not calendar days, so the divisor is the standard working day: 37.5 ÷ 7.5 = 5 days, or 37.5 ÷ 8 = 4.6875 days on an eight-hour standard. Use hours to days when you mean elapsed calendar time, such as an outage duration or a shipping window, and divide by the contractual working day when you mean labor.

Age in days from age in years. Multiplying 30 years by 365.2425 gives 10 957.3 days, which is close but not the answer. The mean year is built to track the calendar over the long run, so the running error stays under roughly a day, but it drifts up and down depending on how many February 29ths fell inside that particular span. For anything that has to be exact, such as a legal age threshold, a clinical dosing window, or an insurance eligibility date, count from the actual date of birth to the actual date rather than converting the rounded number of years. Use years to days for estimates and scale checks, not for cutoffs.

Assuming every day has 24 hours. Two separate things break this. A day that crosses a daylight-saving transition runs 23 or 25 hours in local time, which matters for shift schedules and overnight billing. Separately, UTC is kept within ±0.9 s of Earth’s actual rotation by inserting leap seconds: 27 have been added since 1972, and UTC now runs 37 seconds behind TAI (NIST, Leap Seconds). A conversion factor of 86 400 s per day is right for durations and wrong for timestamps that span one of those events.

Frequently Asked Questions

My contract says 24 months and the invoice says 730 days. Do these match?

They usually don’t. Twenty-four months from a start date is two calendar anniversaries, which is either 730 or 731 days depending on whether a February 29 landed in the window. If the difference of a day changes anything, such as a renewal notice deadline or a warranty expiry, the date arithmetic governs. The day count describes that result; it doesn’t define it.

How many working days are in a year?

There’s no conversion factor for this, because it depends on your jurisdiction’s public holidays and your organization’s leave policy. The usual starting point is 52 weeks × 5 days = 260 days, then subtract public holidays and leave. Any number you see quoted as “the” figure has assumptions baked into it that may not be yours.

I need to convert a server uptime of 10 000 hours into something a manager can read.

Divide by 24 to get 416.67 days, then by 365.2425 to get about 1.14 years. Report it as roughly 13.7 months or “just over a year” rather than the decimal. The precision in 1.14 years is fake once you’ve routed through an averaged year, and the rounded phrasing admits it.

Why does a leap second exist if it’s so inconvenient?

Because Earth’s rotation isn’t perfectly regular, and atomic clocks are. Without the occasional inserted second, civil time would drift away from the position of the sun. The ±0.9 s tolerance NIST maintains is what keeps the clock on your wall agreeing with the sky.

Is 365.25 wrong, then?

Not wrong, just a different convention. It was the Julian calendar’s leap rule and it survives in astronomy as a defined constant. It overshoots the seasons by about 11 minutes a year, which is the drift the Gregorian reform was introduced to fix. For a rough age or timeline estimate the difference between 365.25 and 365.2425 is invisible; for anything spanning centuries it isn’t.

Practical takeaway

1. Seconds, minutes, hours, days, and weeks convert exactly. Use the factors without hesitation.
2. Months and years don’t. Before converting either one, ask whether the question is about a duration (use the average) or a date (use date arithmetic and don’t convert at all).
3. When converting hours to days, decide first whether you mean a 24-hour calendar day or a working day. They are different units that share a name.
4. State your year. If a figure depends on whether a year is 365, 365.25, or 365.2425 days, say which one you used, the way NIST’s tables do.

For the conversion factors behind the time units here and how we check them, see how we verify conversion factors.