24-Hour Clock
24-hour clock
The 24-hour clock is a timekeeping convention that expresses the time of day from 0:00 to 23:59, eliminating AM/PM ambiguity and serving as the standard in military, aviation, medical, and IT contexts.
Glossary
Key terms about time and timezones explained
AM/PM (Ante Meridiem / Post Meridiem)
am pm
AM (ante meridiem) and PM (post meridiem) are Latin-derived abbreviations used in the 12-hour clock system to distinguish the first half of the day (before noon) from the second half (after noon).
Atomic Clock
atomic clock
An atomic clock measures time by using the quantum-mechanical transition frequency of atoms as its reference, achieving accuracies on the order of one second in hundreds of millions of years and forming the backbone of modern timekeeping.
BIPM (International Bureau of Weights and Measures)
bipm
BIPM (Bureau International des Poids et Mesures) is the international organization headquartered near Paris that computes and maintains International Atomic Time (TAI) and Coordinated Universal Time (UTC), coordinating measurement standards worldwide.
Chronotype
chronotype
Chronotype refers to an individual's natural preference for the timing of sleep and activity, broadly classified as morning type, evening type, or intermediate type, and largely determined by genetics.
Circadian Rhythm
circadian rhythm
A circadian rhythm is an approximately 24-hour biological cycle that governs the sleep-wake pattern, body temperature fluctuations, and hormone secretion in living organisms.
DSN (Deep Space Network)
dsn
The Deep Space Network (DSN) is NASA's array of three ground stations spaced around the globe that provides continuous communication with interplanetary spacecraft and performs precision timekeeping.
Daylight Saving Time
dst
The practice of advancing clocks by one hour during warmer months to extend evening daylight.
Epoch
epoch
A fixed reference point in time from which a system measures elapsed time, most commonly January 1, 1970 for Unix systems.
Follow-the-Sun
follow-the-sun
Follow-the-Sun is an operational model in which teams distributed across different time zones hand off work during their respective daytime hours to provide continuous 24-hour service or development without overnight shifts.
GMT (Greenwich Mean Time)
gmt
The mean solar time at the Royal Observatory in Greenwich, London, historically used as the international time reference.
GPS Time
gps time
GPS Time (GPST) is the continuous time scale used internally by the Global Positioning System, originating on January 6, 1980 and never incorporating leap seconds.
Gregorian Calendar
gregorian calendar
The Gregorian calendar is the solar calendar promulgated by Pope Gregory XIII in 1582, now serving as the de facto international civil calendar used throughout the world.
IANA Time Zone Database
iana tzdb
The IANA Time Zone Database (tz database) is the standard reference used by operating systems and programming languages, cataloging current and historical time zone rules - including daylight saving transitions - for regions worldwide.
IERS (International Earth Rotation and Reference Systems Service)
iers
IERS is the international body responsible for monitoring Earth's rotation, determining when leap seconds are needed, and maintaining the global terrestrial and celestial reference frames.
Intercalary Month
intercalary month
An intercalary month is an extra month inserted into lunisolar calendars to realign the lunar year with the solar year, occurring roughly 7 times every 19 years in traditional East Asian and Hebrew calendars.
International Date Line
international date line
The International Date Line is an imaginary boundary running roughly along the 180th meridian in the Pacific Ocean, where the calendar date changes by one day upon crossing.
ISO 8601
iso-8601
An international standard for representing dates and times in an unambiguous, machine-readable format.
Jet Lag
jet-lag
A temporary sleep disorder caused by rapid travel across multiple time zones, disrupting the body's internal clock.
JJY
jjy
JJY is the call sign of Japan's long-wave standard-frequency time signal, operated by NICT from two transmitters - Otakadoya-yama (40 kHz) in Fukushima and Hagane-yama (60 kHz) in Saga - broadcasting JST around the clock.
JST (Japan Standard Time)
jst
JST (Japan Standard Time) is the standard time of Japan at UTC+9, based on the 135th meridian east. Japan does not observe daylight saving time.
Leap Smear
leap smear
Leap smear is a technique that avoids the time discontinuity of a leap second by gradually stretching or compressing each second over a period of several hours to a full day, distributing the one-second adjustment smoothly.
Leap Year
leap year
A leap year is a calendar year containing an extra day (February 29), extending it to 366 days in order to keep the calendar aligned with Earth's orbital period around the Sun.
Leap Second
leap-second
A one-second adjustment applied to UTC to keep it aligned with the Earth's irregular rotation.
Longitude
longitude
Longitude is the geographic coordinate measured as an angle east or west from the Prime Meridian (0 degrees), providing the theoretical basis for time zones and positional reference on Earth.
Melatonin
melatonin
Melatonin is a hormone secreted by the pineal gland that increases in darkness to promote sleepiness, playing a central role in regulating the circadian rhythm.
Meridian
meridian
A meridian is an imaginary semicircle running from the North Pole to the South Pole on the Earth's surface, serving as a reference line for longitude and forming the foundation of the global timekeeping system.
NICT (National Institute of Information and Communications Technology)
nict
NICT is Japan's national research institute responsible for generating, maintaining, and distributing Japan Standard Time (JST), operating a fleet of atomic clocks and transmitting the JJY time signal.
NTP (Network Time Protocol)
ntp
A networking protocol for clock synchronization between computer systems over packet-switched networks.
Optical Lattice Clock
optical-lattice-clock
An optical lattice clock traps atoms in a grid of laser light and measures transitions in the visible-light frequency range, achieving an accuracy of one second in 30 billion years.
PTP (Precision Time Protocol)
ptp
PTP (Precision Time Protocol, IEEE 1588) is a network protocol that achieves microsecond-to-nanosecond time synchronization over LANs, widely used in financial trading and telecommunications infrastructure.
Quartz Clock
quartz-clock
A quartz clock uses the stable vibration (32,768 Hz) produced by the piezoelectric effect of a quartz crystal to measure time, accounting for over 95% of all clocks manufactured worldwide.
Radio-Controlled Clock
radio-controlled-clock
A radio-controlled clock receives longwave time signals broadcast by a national standards authority and automatically corrects its internal quartz timekeeping, maintaining accuracy equivalent to the reference signal when reception succeeds.
Social Jet Lag
social-jet-lag
Social jet lag is the chronic misalignment between a person's social schedule (e.g. work hours) and their internal circadian clock's preferred sleep-wake timing.
Solar Time
solar-time
Solar time is time defined by the apparent position of the Sun. It comes in two forms: apparent solar time, which marks noon when the Sun crosses the local meridian, and mean solar time, which averages out seasonal variations.
Standard Time
standard-time
Standard time is the officially adopted civil time of a region, defined as an offset from UTC and established by law or convention to serve as the basis for all social and commercial activity in that area.
Stratum
stratum
Stratum is the designation for each level in NTP's hierarchical model, ranging from Stratum 0 (atomic clocks and GPS receivers) to Stratum 15, indicating distance from the reference time source.
TAI (International Atomic Time)
tai
TAI is a continuous timescale computed as a weighted average of over 450 atomic clocks worldwide, containing no leap seconds, and serving as the foundation for UTC.
Time Difference
time difference
Time difference is the gap between the standard times of two regions, calculated from their respective UTC offsets. It may vary by season in areas that observe daylight saving time.
Time Dilation
time dilation
Time dilation is a phenomenon predicted by relativity in which time passes more slowly for objects moving at high speed or situated in strong gravitational fields, as observed from a stationary reference frame.
Time Signal
time signal
A time signal is a service that broadcasts the precise current time via audio, radio, or electronic means, including telephone speaking clocks, radio pips, and standard-frequency transmissions.
Timestamp
timestamp
A timestamp is a recorded value representing the exact date and time an event occurred, expressed in formats such as Unix time (epoch seconds) or ISO 8601 strings.
Time Zone
timezone
A region of the globe that observes a uniform standard time for legal, commercial, and social purposes.
Timezone Abbreviation
timezone abbreviation
Timezone abbreviations (JST, EST, CET, etc.) are short codes representing time zones. While convenient for humans, they are ambiguous and should be avoided in software in favor of IANA timezone names.
Unix Timestamp
unix-timestamp
The number of seconds that have elapsed since January 1, 1970 00:00:00 UTC, widely used in computing to represent points in time.
UT1 (Universal Time 1)
ut1
UT1 is a form of universal time derived from the Earth's actual rotation angle. It reflects irregular variations in rotational speed and its divergence from UTC determines when leap seconds are inserted.
UTC (Coordinated Universal Time)
utc
The primary time standard by which the world regulates clocks and time, serving as the basis for civil timekeeping globally.
UTC Offset
utc offset
A UTC offset is the difference in hours and minutes between a region's local standard time and UTC, expressed as +09:00 (Japan) or -05:00 (New York in winter), for example.
Year 2038 Problem
year 2038 problem
The Year 2038 Problem is a software bug in which 32-bit signed integer Unix timestamps overflow on 19 January 2038, causing date/time processing to fail on affected systems.
Zulu Time
zulu time
Zulu time is the military and aviation term for UTC, derived from the NATO phonetic alphabet designation Z (Zulu) for the zero-meridian time zone. The trailing Z in ISO 8601 timestamps originates from this convention.