Time Unit Converter

This tool converts between twenty-three units of time, from femtoseconds through the familiar seconds, minutes and hours up to centuries — plus the units most converters leave out, such as fortnights, shakes, quarters, and the sidereal versions of the day, hour, minute, second and year.

Most time conversion is straightforward because the factors are small whole numbers. The interesting part is the handful of places where a "day" or a "year" does not mean quite what you assume, and those are exactly the cases that produce bugs and scheduling errors.

Solar time and sidereal time are not the same

A mean solar day — the one your clock keeps — is 86,400 seconds. A sidereal day, one rotation of the Earth measured against distant stars rather than against the Sun, is 86,164.0905 seconds. The difference is 235.9 seconds, just under four minutes.

The gap exists because the Earth moves along its orbit while it spins. After one full rotation relative to the stars, it has to turn a little further for the Sun to return to the same position in the sky. Over a year those four minutes accumulate to exactly one extra rotation, which is why there is one more sidereal day in a year than there are solar days.

This matters for telescope tracking, satellite work and astronomical timekeeping, and nowhere else. If you did not come here for that, use the mean solar entries.

Which year did you mean?

The tool offers three, and they differ by enough to matter over long spans:

• Calendar year — 365 days, or 366 in a leap year
• Tropical year — 365.24219 days, the time from one March equinox to the next. This is what the Gregorian calendar approximates with its leap year rules.
• Sidereal year — 365.25636 days, one orbit relative to the fixed stars, about twenty minutes longer than the tropical year.

Ordinary work wants the calendar year. Anything involving seasons over centuries wants the tropical year, which is why the leap year rule skips three leap days every four hundred years.

A worked example: a shift pattern

You are costing a rota of 12-hour shifts and need the annual total in hours.

1. One shift is 12 hours = 43,200 seconds.
2. A calendar year is 365 × 86,400 = 31,536,000 seconds.
3. 31,536,000 ÷ 43,200 = 730 shifts of coverage to staff a position around the clock for a year.

Simpler ones come up constantly: 90 minutes is 5,400 seconds; a fortnight is 14 days or 1,209,600 seconds; a decade is 10 calendar years.

How to use it

1. Enter the duration you have.
2. Select its unit — if you see both a mean solar and a sidereal option, you almost certainly want mean solar.
3. Select the unit you want.
4. The converted value appears straight away.

Duration is not the same as a date calculation

This converter handles durations — how long something lasts. It does not calculate the interval between two calendar dates, and the two are genuinely different problems.

Converting a duration is pure arithmetic: 90 days is always 7,776,000 seconds. Working out the gap between two dates is not, because calendars contain months of unequal length, leap days, and in most countries daylight saving transitions that make one day of the year 23 hours long and another 25. If you need the number of days between two dates, use a date calculator rather than multiplying.

The very small units

At the bottom of the list are the millisecond, microsecond, nanosecond, picosecond and femtosecond, each a thousand times smaller than the last. They come up in different fields: milliseconds in web performance and network latency, microseconds in audio and high-frequency trading, nanoseconds in processor clock cycles and the propagation delay of a signal along a cable, picoseconds and femtoseconds in laser physics and chemical reaction timing. Light travels roughly 30 centimetres in a nanosecond, which is a useful anchor when reasoning about signal timing in hardware.

Frequently asked questions

How many seconds are in a day?

86,400 for a mean solar day. Note that this is a convention: occasional leap seconds mean a handful of real days have contained 86,401.

What is a shake?

Ten nanoseconds. It comes from nuclear physics, where it is roughly the time between stages of a fission chain reaction, and the name is from "two shakes of a lamb's tail".

How long is a fortnight?

Fourteen days. The word is a contraction of "fourteen nights" and is still standard in British and Australian English for a two-week period.

Why is a month listed as a mean calendar month?

Because calendar months vary from 28 to 31 days, a single conversion factor has to be an average — 30.4375 days, one twelfth of a 365.25-day year. Use it for rough planning, not for anything where the specific month matters.

What is a quarter here?

Three months, the standard business reporting period. As with months, it is an average rather than a specific quarter's actual length.

Related tools

• Age Calculator — for the date arithmetic this converter deliberately does not do.
• Speed Converter — distance over time, so the units meet here.
• Power Converter — energy over time, same reason.
• Byte/Bit Converter — useful alongside this for transfer-rate calculations.