Time Converter

Which Time Units Are Exact?

Some time units are fixed and some are not. A minute is exactly 60 seconds, an hour exactly 3,600 seconds and a week exactly 168 hours, so those convert cleanly. Months and years are calendar periods: a month can last 28 to 31 days, and a year 365 or 366. Converting them needs a stated convention, such as a particular month or an agreed average.

Common time periods in seconds and hours
Period	Seconds	Hours	Fixed length?
1 hour	3,600	1	Yes
1 day	86,400	24	Yes, as a duration
1 week	604,800	168	Yes
1 fortnight (14 days)	1,209,600	336	Yes
Month of 28 days	2,419,200	672	No, months vary
Month of 30 days	2,592,000	720	No
Month of 31 days	2,678,400	744	No
Year of 365 days	31,536,000	8,760	No
Leap year of 366 days	31,622,400	8,784	No
Julian year (365.25 days)	31,557,600	8,766	Yes, as a defined convention used in science
Average Gregorian year (365.2425 days)	31,556,952	8,765.82	An average over the 400-year calendar cycle

The fixed units are the ones the two time converters here rely on. The weeks to hours converter multiplies by 168 (two weeks is 336 hours), and the hours to weeks converter divides by 168, so a result promised "within 24 hours" is 24 ÷ 168 = about 0.143 of a week.

One more trap is the day. A duration of 24 hours always contains 86,400 seconds, but a local calendar day does not always contain 24 clock hours. When daylight saving begins, clocks skip an hour, and when it ends an hour is repeated, so adding 24 hours to a timestamp is not always the same as moving to the same clock time on the next date. Before converting, decide whether the value is a fixed duration or a calendar period.

Time Units from Nanoseconds to Millennia

The second is the SI base unit of time. Since 1967 it has been defined by a property of the caesium-133 atom: exactly 9,192,631,770 periods of the radiation corresponding to a transition in its ground state. Every other unit in the list below is a multiple or fraction of it, or a calendar convention built on top.

Common time units and typical uses
Unit	Size	Typical use
Nanosecond (ns)	10⁻⁹ s	Processor timing, electronics, networking, light
Microsecond (µs)	10⁻⁶ s	Electronic signals, computer operations, cameras
Millisecond (ms)	10⁻³ s	Response times, network latency, audio, sports timing
Second (s)	1 s	Clocks, science, electronic systems
Minute (min)	60 s	Short activities, videos, journeys, appointments
Hour (h)	3,600 s	Working time, travel, energy use, schedules
Day (d)	86,400 s	Daily schedules, as a fixed duration
Week	7 days	Work schedules, holidays, project plans
Fortnight	14 days	Payroll and scheduling, common in British English
Month	28 to 31 days	Calendar planning, billing, ages
Year (y)	365 or 366 days	Calendar periods, or a defined year such as the 365.25-day Julian year
Decade	10 years	History, demographic change and long-term trends
Century	100 years	Historical dates and long time spans
Millennium	1,000 years	Historical, archaeological and geological timescales

Three Stories Behind Time Measurement

Why an Hour Has 60 Minutes

Modern timekeeping wasn't invented at one moment. Ancient Egyptian traditions helped establish the division of day and night into hours, and Babylonian astronomers used a number system based on 60. Later astronomers inherited that system and used sexagesimal fractions when recording angles and observations.

Sixty is practical because it divides evenly by 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30, which makes it easy to split an hour into halves, quarters and thirds without awkward fractions. The words minute and second originally meant the first and second small divisions of an hour. The result is the system still in use: 1 hour = 60 minutes and 1 minute = 60 seconds. Decimal time has been proposed more than once, but the older system is too deeply embedded in clocks and schedules.

Railways and a Falling Ball: Making Time Shared

Before standard time, towns set their clocks by local solar time, so places farther east reached noon earlier. That was manageable when journeys were slow. Railways needed coordinated timetables, so British railway companies adopted Greenwich Mean Time, and according to Royal Museums Greenwich most were using it by the 1840s. The Earth turns through about 360 degrees in 24 hours, an ideal 15 degrees of longitude per hour, although real time-zone boundaries follow borders and economic ties, not exact lines.

Ships needed the time too, because a marine chronometer helps determine longitude only if it was set correctly. In 1833 a time ball was installed at the Royal Observatory in Greenwich. It rose before 1 p.m. and dropped at exactly 1 p.m., giving ships on the Thames a visible signal for checking their chronometers. The Greenwich Time Ball still operates: it rises halfway at 12:55 p.m., reaches the top at 12:58 p.m. and falls at 1 p.m.

The 61-Second Minute: Leap Seconds

Atomic clocks keep an extremely stable time scale, but the Earth does not rotate perfectly regularly. Leap seconds were introduced in 1972 to keep Coordinated Universal Time (UTC) close to time based on the Earth's rotation. In a positive leap second, the end of the day counts 23:59:59 → 23:59:60 → 00:00:00, so one minute lasts 61 seconds. That is different from a leap day, which realigns the calendar with the Earth's orbit.

Twenty-seven leap seconds have been added since 1972, the most recent on 31 December 2016, and no negative leap second has ever been used. Because many computer systems assume every minute has exactly 60 seconds, each one caused problems. In November 2022 the General Conference on Weights and Measures decided to stop using leap seconds by or before 2035. The NIST explanation of leap seconds covers the details.

A Microsecond Is About 300 Metres in GPS

GPS satellites transmit signals containing their position and a precise time. A receiver compares when a signal was sent with when it arrived, and because radio signals travel at the speed of light, the travel time gives a distance. Light covers 299,792,458 metres in one second, so in one microsecond it covers about 299.8 metres:

0.000001 s × 299,792,458 m/s ≈ 299.8 m

A timing error that looks tiny on a clock is therefore a large error in position. Satellites carry atomic clocks, and a receiver uses signals from at least four satellites to solve for three-dimensional position and its own clock offset. See GPS.gov and NIST for official explanations.