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CALENDAR
Defining A Day
A "day" is one full sun cycle. "Day time" is when the sun is visible, from sunrise to sunset. "Night time" is when the sun is not visible, from sunset to sunrise.
- 1 full sun cycle = 1 day
- 1 day = 24 hours
- 1 hour = 60 minutes
- 1 minute = 60 seconds
Small Time Units
- 1 second = 1,000 milliseconds (ms)
- 1 second = 1,000,000 microseconds (us)
- 1 second = 1,000,000,000 nanoseconds (ns)
- 1 decisecond = 0.1 seconds
- 1 centisecond = 0.01 seconds
- 1 millisecond = 0.001 seconds
Three Meanings Of Day
A day can mean a period of time, a calendar day, or the daylight part of the day. A day period is one sun cycle with no required starting point. A calendar day is also one sun cycle, but it is defined as starting at 12:00 AM midnight.
The sun defines the day cycle. The moon defines the month cycle.
Month, Season, And Year
In a 13-month moon-based model, 13 cycles of 28 days equals 364 days, plus one extra day to reach 365 days. The sun's position in the sky defines the seasons and the year, because the sun can appear higher or lower through the year.
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Other Clocks
UTC TimeXX:XX UTC
12-Hour TimeXX:XX XX XXX
24-Hour TimeXX:XX XXX
Roman TimeXX:XX
Greek TimeXX:XX
Arabic TimeXX:XX
Japanese TimeXX:XX
Kanji TimeXX:XX
Chinese TimeXX:XX
Devanagari TimeXX:XX
Thai TimeXX:XX
Gaelic TimeXX:XX
Binary TimeXXXXXX XXXXXX
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Time Information
Where our units of time came from, how clocks keep a steady rhythm, and how modern devices agree.
How Time Works
Time tells us the order and duration of events. A clock does not create time; it counts a process that repeats at a steady rate, while a calendar groups those counts into days, months, and years.
1 day24 hours
→
1 hour60 minutes
→
1 minute60 seconds
Who divided the day?
There was no single inventor. Ancient Egyptian timekeepers divided daylight into 12 parts and night into 12 parts, creating the basis of the 24-hour day. The first “hours” changed length with the seasons; equal-length hours became practical much later.
Why 60 minutes and 60 seconds?
Babylonian mathematicians used a base-60 number system. Greek and Hellenistic astronomers later used those sexagesimal divisions in calculations. Over centuries, that tradition became 60 minutes in an hour and 60 seconds in a minute.
Why was timekeeping needed?
Communities needed shared time for farming, markets, worship, government, navigation, and night watches. Railways, factories, telecommunication, and global travel later made standardized time zones and closely synchronized clocks essential.
From the sky to atomic time
Sun and starsNatural cycles marked days, seasons, and night hours.
Flow and burningWater, sand, candles, and incense measured intervals.
Mechanical clocksEscapements, gears, and pendulums counted regular motion.
Electronic clocksQuartz vibrations provided a compact, dependable frequency.
Atomic standardsAtomic transitions now define the second and support UTC.
How a Crystal Produces Time
Quartz is piezoelectric: electricity makes a carefully cut crystal flex and vibrate, and the vibration creates an electrical signal in return. A watch circuit keeps that vibration going, counts its steady frequency—commonly 32,768 vibrations each second—and repeatedly divides the count by two until it has one pulse per second.
The crystal is the clock’s oscillator, or repeating rhythm. Electronics count the rhythm and move the hands or update the display. Temperature, crystal shape, and aging can change the frequency slightly, which is why quartz clocks still drift and periodically resynchronize. Atomic clocks follow the same basic count-a-rhythm idea, but use an atomic resonance that is far more stable.
Modern Civil Time
No single company sets world time. UTC is an international standard assembled from atomic-clock measurements, then national laboratories and network services distribute practical versions of it.
Atomic clocks
About 450 clocks at roughly 85 laboratories contribute measurements to the international system.
BIPM computes UTC
The International Bureau of Weights and Measures combines those clocks to produce TAI and UTC.
Time is distributed
In the United States, NIST and the U.S. Naval Observatory maintain and distribute official U.S. time. The IERS monitors Earth rotation and announces leap seconds.
Your device syncs
Your operating system checks a network time server. This page then reads the clock supplied by your browser and device—it does not contact an atomic clock directly.
Network Time Protocol
NTP is an open Internet protocol that lets a device compare its clock with one or more network time servers and correct the difference.
Who manages NTP?
No single company controls it. The IETF Network Time Protocols Working Group maintains the Internet standards, and IANA maintains the protocol registries. The NTP Project, part of the nonprofit Network Time Foundation, develops and maintains the open-source reference implementation. The servers themselves are operated by many national laboratories, universities, technology providers, organizations, and volunteers.
What this page reads
On a typical non-domain Windows computer, Windows Time commonly uses time.windows.com, though organizations can configure a different source. This page reads the time supplied by your browser and operating system; it does not query an NTP server itself.
How much clocks drift
A quartz wristwatch may be off by about 15 seconds per month. Many household quartz clocks stay within about one second per day, while some drift several seconds per day. Temperature, age, power, and component quality all matter.
How often to resynchronize
There is no universal interval. Connected devices normally resync automatically and more often when their error grows. Daily or regular automatic checks are usually adequate for an everyday display; precision systems use frequent or continuous discipline. Offline devices can keep accumulating error.
One atomic second
is defined using exactly 9,192,631,770 periods of a transition in the cesium-133 atom.
How to Read an Analog Clock
Short hand = hours. Read the number it has most recently passed. The hour hand moves gradually toward the next number as the minutes pass.
Long hand = minutes. Count clockwise from 12: each small mark is one minute and each numbered step is five minutes. At 12 it is :00, at 3 it is :15, at 6 it is :30, and at 9 it is :45.
Example: 3:30. The minute hand points to 6 and the hour hand sits halfway between 3 and 4. If there is a thin second hand, each small mark is one second. A standard 12-hour clock does not distinguish AM from PM.
Timekeeping Devices
Every clock combines a repeating or predictable process with a way to count and display the result.
Sundial
A raised pointer casts a shadow across hour markings. It shows local apparent solar time, so it needs sunlight, correct orientation, and seasonal correction to compare with civil clock time.
Hourglass
Sand flowing through a narrow neck measures a fixed interval rather than the time of day. Grain size, humidity, the opening, and whether the glass is level all affect its rate.
Water Clock
A clepsydra marks time by water entering or leaving a calibrated vessel. It works indoors and at night, but changing water pressure, temperature, and flow can alter its accuracy.
Pendulum Clock
An escapement releases the gears one step for each regular swing. Pendulum length, gravity, temperature, and keeping the case level influence how quickly the clock gains or loses time.
Cuckoo Clock
A traditional weight-driven pendulum clock. On the hour, gears operate bellows and a small mechanical bird; the familiar call is the alarm mechanism, not the timekeeping mechanism.
Quartz Clock
A battery-powered circuit counts a quartz crystal’s regular vibration—commonly 32,768 cycles per second—and divides it into one-second pulses. Temperature and crystal aging still cause drift.
Candle Clock
Evenly spaced marks on a candle estimate elapsed time as the wax burns downward. Drafts, wick length, and uneven wax make it useful for rough intervals rather than exact time.
Marine Chronometer
A highly stable portable mechanical clock let sailors compare local noon with the time at a reference meridian, making accurate longitude calculations possible at sea.
Atomic Clock
Electronics tune a signal to the resonance of atoms such as cesium. Counting that exceptionally stable frequency produces the standards used for navigation, communications, and UTC.
Custom Time Interval
Choose a starting and ending date and time to see where today falls within the interval.
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