| Reference | Days (d) | Milliseconds (ms) |
|---|---|---|
| A blink of an eye | 0.00000115741 d | 100 ms |
| One heartbeat at rest | 0.00000960648 d | 830 ms |
| Half of a football match | 0.03125 d | 2700000 ms |
| A full day | 1 d | 86400000 ms |
The day is a unit of time equal to 24 hours, or 86,400 seconds. Its symbol is d. It is accepted for use with the SI without being an SI unit, and it is the only common time unit that corresponds directly to a physical cycle people can observe without instruments.
There is more than one kind of day, and the difference matters in astronomy. The solar day is the time between successive noons, when the Sun returns to the same position in the sky. The sidereal day is the time the Earth takes to rotate once relative to the distant stars, and it is shorter by about three minutes and fifty-six seconds. The gap exists because the Earth also moves along its orbit, so it must turn slightly more than a full rotation to bring the Sun back overhead.
The solar day is not constant either. The Earth's orbit is elliptical and its axis is tilted, so the interval between noons varies through the year by up to about thirty seconds either way. The mean solar day averages this out, and the equation of time describes the difference, which is why a sundial can run a quarter of an hour ahead of or behind a clock depending on the season.
The civil day of exactly 86,400 seconds is a convention rather than a measurement. The Earth's rotation is gradually slowing through tidal friction with the Moon, at a rate that lengthens the day by roughly two milliseconds per century, and it also varies irregularly with atmospheric and core motions. Leap seconds have been inserted since 1972 to keep clock time within a second of solar time, and the practice is due to be discontinued by 2035 in favour of allowing a larger drift.
Beyond timekeeping the unit organises everything periodic in ordinary life: employment and rental contracts, interest calculations, drug dosing schedules, weather statistics, shipping and delivery estimates, and the whole apparatus of calendars. Biology has its own version in the circadian rhythm, an internal cycle of very nearly twenty-four hours that persists even in the absence of daylight cues.
Astronomy uses the Julian day, a continuous count of days since a fixed epoch in 4713 BC, precisely because calendars are irregular and a simple running total is easier to compute with.
One day equals 24 hours, 1440 minutes, or 86,400 seconds.
The millisecond is a unit of time equal to one thousandth of a second. Its symbol is ms. It is the shortest interval that people encounter by name in everyday life, largely because computing and networking made it a household measurement.
Human perception sets the scale that makes it interesting. The eye fuses separate images into motion somewhere around forty milliseconds apart, which is why cinema at twenty-four frames a second works. Audio is far less forgiving: a delay of more than about ten milliseconds between a musician's action and the sound they hear is enough to disrupt playing, and stereo direction is judged from arrival-time differences between the ears of well under one millisecond.
Network latency is quoted in milliseconds and is the number that determines whether a connection feels responsive. Light travels about three hundred kilometres in a millisecond in fibre, so intercontinental round trips have an irreducible floor: London to New York cannot go below roughly sixty milliseconds no matter how much is spent on equipment. Competitive online gaming, financial trading and video calling are all shaped by that limit.
Reaction time is measured in the same unit. A trained sprinter responds to a starting gun in around 150 milliseconds, and athletics rules treat anything below 100 milliseconds as a false start on the grounds that no human can react faster. Simple visual reaction times in a laboratory typically fall between 200 and 250 milliseconds.
Computing uses milliseconds as the natural unit for anything involving storage or human interaction. A mechanical hard disk seeks in several milliseconds while a solid-state drive answers in a fraction of one, a difference of two orders of magnitude that explains most of the change in how computers feel over the last two decades. Timestamps in most programming environments count milliseconds since the start of 1970.
Photography borrows it for shutter speeds, where a thousandth of a second is a common setting for freezing motion, and physiology uses it for the duration of a nerve impulse, which lasts one to two milliseconds.
One millisecond equals 0.001 seconds, 1000 microseconds, or one 3,600,000th of an hour.