Nanoseconds to fortnights, exactly — and months and years handled as what they are, which is a range and a choice rather than two more rows in the list.
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Months and years are not in this list, and their absence is the point. Every unit here has exactly one length. A month does not, and a year has four in common use, so each gets its own panel below where the convention can be named.
1 day
86,400 seconds
1 d · 0 h · 0 min · 0 s
Every unit that has one length
Ordered by size — nanoseconds to fortnights, all exact
Unit
Value
Note
nanosecond
8.6400e+13
Light travels about 30 cm in one.
microsecond
8.6400e+10
A million to the second.
millisecond
86,400,000
Human reaction time is a few hundred.
second
86,400
The SI base unit, defined by a caesium transition rather than by the Earth.
minute
1,440
Sixty seconds, always — minutes have never taken a leap second.
hour
24.000
Sixty minutes. Unchanged since Babylon, give or take.
day
1.000
Exactly 86 400 seconds by convention. A few real UTC days have had 86 401.
week
0.142857
Seven days, with no exceptions anywhere — the only large unit that is genuinely unambiguous.
fortnight
0.071429
Fourteen days. Still the standard pay period across the UK, Australia and India.
The week is the quiet hero of this table: seven days everywhere, in every calendar system that has one, with no exceptions and no leap week. It is the only unit above the day that never needs a convention attached — which is why so much scheduling that could have used months uses fortnights and weeks instead.
Which year did you mean
The same span under four definitions of a year
Convention
Days each
Total days
Used for
Common year
365
3,650
Casual arithmetic, and most "seconds in a year" answers.
Tropical year
365.24219
3,652
Equinox to equinox — the year the calendar is trying to track.
Gregorian year
365.2425
3,652
The average length of a calendar year, leap rules included.
Julian year
365.25
3,653
Astronomy, and the year inside a light-year.
A Julian year and a common year are exactly six hours apart — a quarter of a day, which is where the leap day comes from. Over 10 years that is 2.500 days of difference, produced entirely by which definition someone picked and did not mention.
How long is a month
Shortest real run89 daysconsecutive calendar months
Longest real run92 daysa 3-day spread
Average Gregorian91.311 dayscorrect on average, correct for no month
30-day convention90 daysinside the real range
These are computed by sliding a window across real consecutive months, not by multiplying. Three months from December is 89 days and from March is 92, and neither is wrong. It is also why finance uses a 30-day month and simply accepts the error: twelve of them is 360 days, five short of any real year, which is a known and deliberate approximation rather than a mistake.
The conventions in use, and what each is for
Convention
Days
Note
30-day month
30.000000
The convention in finance, interest accrual and most contracts. Convenient, and never exactly right.
Average Gregorian month
30.436875
A twelfth of the average calendar year — 30.436875 days. Correct on average and correct for no particular month.
Average Julian month
30.437500
A twelfth of the Julian year, 30.4375 days. Used where the Julian year already is.
Three places the arithmetic has a footnote
Seconds in a common year31,536,000or 31,536,001 in a leap-second year
A rotation of the Earth86,164.0905 s3.93 minutes short of a solar day
Runs from2001–2100there was no year zero
Three separate footnotes, none of which changes an everyday answer and each of which changes a precise one. A day is 86 400 seconds by convention — 27 real UTC days since 1972 have had 86 401, and the 2022 decision to stop inserting leap seconds by 2035 is why that list has an end. The Earth turns in 23 h 56 min: the extra four minutes of a solar day come from the orbit rather than the spin, which is why a given star rises four minutes earlier each night. And centuries are numbered from 1, so the 21st began in 2001 — the same off-by-one that made the year 2000 an argument.
What this converter covers
Nine exact units in one table, months as a real calendar range, four year conventions side by side, and the three footnotes that only matter when they do.
Nanoseconds, microseconds, milliseconds, seconds, minutes, hours, days, weeks and fortnights
Months as a range of real consecutive calendar months, not a multiplication
Common, tropical, Gregorian and Julian years, side by side
Leap seconds, and why they are ending
Sidereal against solar days, and century numbering
Nine exact units Months as a range Four different years ns to fortnights
Free, no signup — exact by definition, not an estimate.
Updated 8 September 2026
At a glance
Formula shown
Exact: 1 d = 86 400 s, 1 wk = 7 d · Not exact: a month, and the choice of year
Scenario support
Three months is 89 to 92 days · a Julian and a common year are exactly six hours apart
Educational estimate
Planning support from the values you enter — not professional advice.
The units that have one length
Nine units of time convert into one another with no argument at all: the nanosecond up through the fortnight. Each is a fixed multiple of the second, and the second is defined by a caesium transition rather than by anything the Earth does.
The week deserves particular notice. Seven days, everywhere, in every calendar system that uses one, with no leap week and no local variation. It is the only unit above the day that never needs a convention attached.
That is not a small property. It is why payroll runs fortnightly, why sprints are two weeks, and why anything that has to be counted rather than approximated tends to be expressed in weeks even when months would read more naturally. A month is easier to say and harder to use.
Everything above the fortnight leaves this list. Months, quarters, years, decades and centuries all need something stated before they mean a duration, and the rest of this page is about what.
A month is a range
There is no number of days in a month. There are four numbers, and which one applies depends on which month, and on the year.
That does not average away as neatly as it sounds either, because real months come in a fixed order:
Consecutive calendar months, shortest and longest runs
Span
Shortest
Longest
At 30 days each
One month
28 days
31 days
30
Three months
89 days
92 days
90
Six months
181 days
184 days
180
Twelve months
365 days
366 days
360
Three months from the first of December is 89 days. Three months from the first of March is 92. Both are three months, and a calculator that answers “91.31 days” has answered a question nobody asked.
The last row is the one worth remembering: twelve 30-day months is 360 days, five short of any real year. Finance uses that convention anyway — it is baked into interest accrual, bond conventions and a great many contracts — because a consistent approximation is more useful there than an accurate variable. The error is known, deliberate and priced in.
So the calculator above reports a range and names the conventions rather than picking one. If a specific convention governs your case, it will be written into the contract or the statute, and that is the one to use.
Four years, six hours apart
A year is better behaved than a month and still not a single quantity. Four definitions are in ordinary use.
The four years, shortest first
Year
Days
Where it is used
Common
365
Casual arithmetic, and most answers to “seconds in a year”
Tropical
365.24219
Equinox to equinox — what the calendar is chasing
Gregorian
365.2425
The average calendar year, leap rules included
Julian
365.25
Astronomy, and the year inside a light-year
The common and Julian years are exactly six hours apart — a quarter of a day, which is precisely why there is a leap day at all. The Gregorian refinement exists because a quarter day is slightly too much: skipping the leap year in three centuries out of four brings the average from 365.25 down to 365.2425, close enough to the tropical year to drift by only a day in three thousand.
Over a single year the choice is invisible. Over a century it is a day and a half. And in any calculation that multiplies — compounding, ageing, orbital periods — the difference accumulates in a direction nobody chose, because whoever wrote the formula picked one and did not say.
A day is 86,400 seconds by agreement
Every conversion on this page treats a day as exactly 86,400 seconds. That is a convention, not a measurement, and it is occasionally false.
The Earth’s rotation is not perfectly steady — tides slow it, and mass moving inside and on it speeds it up and slows it down unpredictably. The atomic second does not care. So civil time, UTC, is kept within a fraction of a second of the Earth by inserting an extra second when the gap grows too large.
Twenty-seven of those have been added since the system began in 1972, most recently at the end of 2016. On each of those days, UTC really did have 86,401 seconds, and every piece of software that assumed otherwise had an interesting evening.
This is ending. In 2022 the General Conference on Weights and Measures resolved to increase the tolerated gap between atomic time and Earth rotation by 2035, which in practice stops leap seconds being inserted at all. The convention will then be true — until the accumulated drift has to be dealt with in some larger unit, which is a problem deliberately left to a later century.
For anything a converter is used for, none of this matters. It matters for timestamps, financial sequencing, and anything that compares two instants precisely, which is exactly the category of software that keeps discovering it.
Rotation is not a day
One last place where two things share a name. The Earth does not take 24 hours to turn once. It takes 23 hours, 56 minutes and 4 seconds.
The missing four minutes are the orbit. While the Earth spins, it also moves along its path around the Sun, so after one full rotation it has to turn a little further for the Sun to be overhead again. That extra turn is the difference between a sidereal day — one rotation, measured against the stars — and the solar day the clock keeps.
The consequence is visible from a garden. A given star rises about four minutes earlier each night, which adds to roughly two hours a month and a full cycle over a year. It is why constellations are seasonal, and why the same sky at the same clock time in July and January is a different sky.
Astronomers keep sidereal time for exactly this reason: a telescope pointed by sidereal time stays on the same star, while one pointed by civil time drifts by a degree an hour. It is not an obscure alternative clock — it is the one that matches what the sky is doing.
Related calculators
Other time, rate and scale tools:
Time ZoneConvert a date and time between EST, IST, GMT, UTC, CET, PST, or any IANA time zone, with live daylight-saving handling and a multi-zone world clock view.
Time PercentageWork out what percentage one span of time is of another, or the percentage increase or decrease between an old and a new duration.
Speedmph, km/h, m/s, ft/s and knots on exact factors, with the nautical mile behind the knot explained rather than assumed.
Frequency to WavelengthFrequency, wavelength, photon energy and wavenumber — in vacuum and in the medium, including coax velocity factors.
Light YearLight-years, parsecs, astronomical units and kilometres — all exact by definition — plus the light-travel time and what a 365-day year costs.
AngleDegrees, radians, gradians, arcminutes and arcseconds — and why a spreadsheet’s SIN(90) returns 0.894 rather than 1.
The second comes from the SI definition, the leap seconds and the Earth-rotation figures from the body that measures and announces them, and the end of leap seconds from the conference resolution that decided it. Nothing on this page depends on a calendar authority, because the calendar units are precisely the ones this page declines to reduce to a number.
SI Brochure, 9th edition — the secondBureau International des Poids et Mesures · verified 2026-09-08 · The second as an SI base unit defined by a caesium hyperfine transition rather than by any motion of the Earth, which is what makes every unit in the main ladder exact and leaves the calendar units outside it
International Earth Rotation and Reference Systems ServiceIERS · verified 2026-09-08 · The leap seconds inserted into UTC since 1972 — twenty-seven to date, announced in Bulletin C — and the Earth-rotation measurements behind the difference between a sidereal and a solar day
27th General Conference on Weights and Measures (2022)Bureau International des Poids et Mesures · verified 2026-09-08 · The 2022 resolution on the future of the coordinated universal time scale, which sets out that the maximum difference between UTC and UT1 will be increased no later than 2035 — effectively ending the insertion of leap seconds
Conversion note
This converts durations, not dates. Adding three months to a specific date is calendar arithmetic with its own rules — what 31 January plus one month means is a policy decision that different systems answer differently — and none of that is attempted here. Local time adds daylight saving transitions, which make some local days 23 or 25 hours long even though the underlying duration is unchanged; time-zone rules also change by legislation, sometimes at short notice. The leap-second count is current as of the date shown and the schedule for ending them is set by international resolution rather than fixed, so treat both as subject to revision. The month ranges are computed from the Gregorian calendar; other calendars in current civil and religious use have months of different and sometimes observationally determined lengths. For anything contractual — interest accrual, notice periods, statutory deadlines — the convention that governs is the one written into the agreement or the law, not the one that is most nearly correct.
Published the Time Unit Converter: nanosecond, microsecond, millisecond, second, minute, hour, day, week and fortnight, all exact multiples of the SI second.
Leaves months and years OUT of the unit list on purpose, because every unit in that list has exactly one length and those two do not. Each gets its own panel where the convention can be named.
Reports a month as a RANGE computed by sliding a window across real consecutive calendar months rather than by multiplying: three months is 89 days from December and 92 from March, and neither is wrong.
Records that twelve 30-day months is 360 days, five short of any real year, and that finance uses the convention anyway because a consistent approximation is more useful there than an accurate variable.
Puts the four years in common use side by side -- common 365, tropical 365.24219, Gregorian 365.2425 and Julian 365.25 -- and notes that the shortest and longest are exactly six hours apart, which is where the leap day comes from.
States that a day is 86400 seconds by convention rather than by measurement: 27 real UTC days since 1972 have had 86401, and the 2022 CGPM resolution to widen the UTC-UT1 tolerance by 2035 is why that list has an end.
Separates the Earth's rotation from a day. A sidereal day is 23 h 56 min 4 s, and the missing four minutes are the orbit rather than the spin -- which is why a given star rises four minutes earlier each night.
Notes the century off-by-one, since there was no year zero and the 21st century began in 2001.
Verified by 70 automated cases, asserting that months, years and quarters are absent from the ladder structurally rather than as a special case, that month spans come from the real calendar and always bracket their own average, that the four years are distinct and exactly six hours apart at the extremes, and that the sidereal gap is between 3.9 and 4.0 minutes.
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