Online Tool Store
Time Unit Converter
Convert between time units from nanoseconds to centuries, with the year definition your choice and each unit marked as exact or conventional.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
In plain terms
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| Unit | Value | Definition |
|---|
How to use it
- Enter an amount and the unit it is in.
- Every other unit converts at once, with its definition alongside.
- If the answer involves months or years, pick the year definition you mean.
- Read the plain-language line for a duration you can actually picture.
Where the exactness stops
Most time conversions are definitional and therefore perfectly exact. Then they stop being exact, and most converters do not tell you where the line is:
minute 60 s exact
hour 3,600 s exact
day 86,400 s exact by convention
week 604,800 s exact
fortnight 1,209,600 s exact
────────────────────────────────────
month a convention
year four common definitions
Four different years
common 365 days 31,536,000 s contracts, everyday use
Gregorian 365.2425 31,556,952 s the calendar's mean
tropical 365.24219 31,556,925 s equinox to equinox
Julian 365.25 31,557,600 s astronomy's fixed unit
1,000,000,000 seconds in years
Gregorian 31.688739
common 31.709792 ← 7.9 days further on
The Gregorian mean of 365.2425 comes straight out of the leap rule — a leap year every four years, except centuries, except every fourth century — which averages to 97 leap days per 400 years. The Julian year of exactly 365.25 days is kept as a fixed unit precisely because it never changes, which is what you want when defining a light-year.
The 30-day month is worse than it looks
a twelfth of a Gregorian year 30.43688 days 2,629,746 s
a flat 30-day month 30 days 2,592,000 s
difference 37,746 s ≈ 10.5 hours
Ten and a half hours per month sounds trivial until it is twelve months of billing periods, at which point it is more than five days. Months here are always a twelfth of whichever year you chose, so the table stays internally consistent — twelve of its months add up to exactly one of its years, which a 30-day month never does.
A note on the day
86,400 seconds is a definition rather than an observation. A handful of real days have contained 86,401 seconds because a leap second was inserted to keep clocks aligned with the Earth's rotation, and Unix time skips those entirely. For converting durations this is irrelevant — but it means these figures count calendar days rather than elapsed SI seconds, which matters if you are ever comparing against an atomic clock.
FAQ
How many seconds are in a year?
It depends which year you mean, and the tool makes you choose. A common year of 365 days is 31,536,000 seconds; the Gregorian mean year of 365.2425 days is 31,556,952; the Julian year of 365.25 days is 31,557,600. Astronomers use the Julian year as a fixed unit, calendars follow the Gregorian mean, and contracts usually mean 365.
Does the choice actually matter?
On short spans, no. On long ones it is very visible: a billion seconds is 31.6887 Gregorian years but 31.7098 common years, a difference of nearly eight days. If you are converting a duration in a contract or an SLA, the definition is worth pinning down.
Why is a month not 30 days here?
Because no month is 30 days on average. A twelfth of a Gregorian year is 30.437 days, so treating a month as 30 loses about 10.5 hours every time. That compounds fast — over a year it is more than five days. Months here are derived from the year you selected rather than rounded.
Which units are exact?
Everything from nanoseconds up to fortnights. A minute is exactly 60 seconds, an hour exactly 3,600, a day exactly 86,400, a week exactly 7 days. From months upwards nothing is exact, and the tool marks the difference so you know which figures are definitions and which are conventions.
Does a day really have 86,400 seconds?
By definition here, yes — that is the Unix convention and what every calendar library uses. In reality a handful of days have had 86,401 because a leap second was inserted, so this is a count of calendar days rather than a count of elapsed SI seconds. For durations of any practical length the difference is a rounding error.
Why show a plain-language breakdown?
Because large second counts are meaningless to read. A billion seconds is a number; "31 years, 8 months, 8 days" is a duration you can picture. The breakdown uses the same year definition you chose, so it stays consistent with the table.
How we compare
| Feature | Online Tool Store | Search-box conversion | Other unit converters |
|---|---|---|---|
| Lets you choose the year definition | ✓ | ✗ | ✗ |
| Marks which units are exact | ✓ | ✗ | ✗ |
| Months as a twelfth of a year, not 30 days | ✓ | Usually 30 | Usually 30 |
| Plain-language breakdown | ✓ | ✗ | Sometimes |
| Every unit at once | ✓ | ✗ | ✓ |
| Faster than loading a page | ✗ | ✓ | ✗ |
| Calendar-aware date arithmetic | ✗ | ✗ | ✗ |
A search box will convert hours to minutes faster than this loads. What it will not do is tell you which of four years it used, or that its month was rounded — which is the only reason a converter for time needs to exist at all. For "what date is 18 months from Tuesday", use a date calculator; this converts durations, not dates.