Comprehensive Glossary of Timekeeping & Time Zone Terms

Time zones (see overview, zone map), UTC offsets, and daylight saving practices can be confusing, especially when planning travel, international meetings, or working with global teams. This glossary provides clear explanations of the most important timekeeping terms, from UTC and GMT to sidereal time and standard time offsets (full offsets table). Whether you are a frequent traveler, a student of astronomy, or simply curious about how the world measures time, you’ll find concise and reliable definitions here. For common questions, visit our FAQ.

A

• Advance Time Zone: Region ahead of UTC, e.g., UTC+01:00. For example, Central European Time (CET) is UTC+1.
• Atomic Clock: High-precision clock using atomic oscillations. These are used in GPS satellites and scientific observatories.
• Atomic Time: Time based on atomic clock measurements, far more accurate than solar time.
• Astronomical Time: Time based on celestial body positions, often used for navigation and star tracking.

B

• Barycentric Dynamical Time (TDB): Time standard for solar system dynamics, used in astronomy and planetary science.
• Baseline Time: Reference time for synchronization, often used in experiments and networks.

C

• Chronobiology: Study of biological rhythms and time, e.g., human sleep cycles.
• Chronometer: Precise timekeeping device, often certified. Used historically in ships to determine longitude.
• Civil Time: Time used for civilian purposes, usually the official standard or local time in a country.
• Clock Drift: Gradual deviation of a clock from accurate time, e.g., a wall clock losing 2 minutes a month.
• Clock Skew: Difference in time between two clocks in a system, important in computing.
• Clock Synchronization: Aligning clocks to a common time standard, e.g., using NTP on servers.
• Coordinated Universal Time (UTC): Global time standard based on TAI with leap seconds. UTC is the reference for time zones worldwide.

D

• Daylight Saving Time (DST): Seasonal time adjustment, advancing clocks (usually +1 hour) to extend evening daylight. Example: much of Europe moves clocks forward in March. Learn more: DST guide.
• Diurnal Motion: Daily motion of celestial bodies due to Earth’s rotation, visible as the Sun’s path in the sky.
• Double Summer Time: Advancing clocks by two hours during DST (historical use in the UK during WWII).
• DST Adjustment: Change in clock time for DST transitions, e.g., “spring forward” or “fall back.” See DST rules.
• DST Start/End Date: Specific dates for beginning and ending DST, which vary by country; check our FAQ for common cases.

E

• Ephemeris Time: Time based on planetary and celestial motions, important for astronomy.
• Equation of Time: Difference between apparent and mean solar time, explains why sundials can differ from clocks.
• Equatorial Time: Time measured relative to the celestial equator, used in astronomical calculations.

F

• Fallback (DST Transition): Reverting clocks back in autumn (US/UK: "fall back"). Example: moving from 2:00 AM to 1:00 AM in November in the US; see DST timing.
• Forward Shift (Spring Forward): Advancing clocks in spring for DST (US/UK/Canada); see DST overview.

G

• Geocentric Time: Time relative to Earth’s center, used in astronomy for precise measurements.
• GPS Time: Time system used by GPS, offset from UTC. Essential for navigation and mapping.
• Greenwich Mean Time (GMT): Time standard at Prime Meridian, UTC+00:00. See the page on GMT.

H

• Half-hour Time Zone: Regions offset from UTC by 30 minutes, e.g., India Standard Time (IST) UTC+05:30.
• Horology: Science of time measurement and clockmaking, including watch design.

I

• International Atomic Time (TAI): Time standard based on atomic clocks (no leap seconds); UTC is derived from TAI by adding leap seconds.
• International Date Line (IDL): Line marking calendar day change, roughly 180° longitude. Crossing it changes the calendar date (see examples in our FAQ).
• International Earth Rotation and Reference Systems Service (IERS): Organization managing time and reference systems, decides on leap seconds (related to UTC).
• ISO 8601: International standard for date and time formats, e.g., 2025-09-11T09:00:00Z. See common usage in FAQ.

J

• Julian Date: Continuous count of days since 4713 BCE, used in astronomy and historical research.

L

• Leap Minute (proposed concept): Hypothetical adjustment for long-term timekeeping.
• Leap Second: Second added to UTC to align with Earth’s rotation; the last was added in 2016.
• Leap Year: Year with an extra day (February 29) to align calendar with Earth’s orbit; see examples in the FAQ.
• Legal Time: Officially recognized time in a jurisdiction, usually defined by law (often tied to time zones and DST).
• Local Apparent Time: Time based on the Sun’s position at a location, as shown by sundials.
• Local Mean Time (LMT): Average solar time at a specific longitude, used before standard time zones.
• Local Solar Time: Time based on the Sun’s position in the sky, e.g., solar noon when Sun is highest.
• Longitude-based Timekeeping: Time derived from longitudinal position relative to Prime Meridian, historically vital for navigation.
• Lunar Time: Time based on Moon’s motion, used in lunar calendars like the Islamic calendar.

M

• Mean Solar Time: Average time based on Sun’s daily motion, close to civil time.
• Mean Time: Time based on average solar day length.
• Meridian Time: Time based on a specific meridian’s solar time, historically used locally.
• Metonic Cycle: 19-year cycle aligning lunar and solar calendars, used in calculating Easter.
• Midnight Rule: Standard for starting a new day at midnight, adopted globally.
• Military Time: 24-hour clock format, e.g., 14:00 for 2:00 PM. Common in transport and armed forces; see conversions in FAQ.

N

• Nautical Time: Time used at sea, often tied to longitude and ship’s location.
• Network Time Protocol (NTP): Protocol for synchronizing clocks over networks, critical for the internet; closely related to UTC distribution.

O

• Observatory Time: Precise time kept at observatories for astronomy.
• Offset Time Zone: Time zone defined by its UTC offset (see Standard Time Offsets), e.g., UTC+09:00 (Japan).

P

• Prime Meridian: 0° longitude, basis for GMT/UTC, located in Greenwich, London.

R

• Railway Time: Standardized time introduced for railway schedules (UK, 19th century), precursor of time zones.

S

• Sidereal Day: Time for Earth to complete one rotation relative to stars, about 23h 56m.
• Sidereal Time: Time based on Earth’s rotation relative to fixed stars, used in astronomy.
• Solar Noon: Time when the Sun is at its highest point in the sky.
• Solar Time: Time based on Sun’s position, e.g., measured by sundials.
• Standard Meridian: Reference meridian for a time zone’s base time.
• Standard Time: Official time when DST is not in effect; see offsets.
• Summer Time: UK/Canada term for DST, advancing clocks in summer.
• Summer/Winter Time: Seasonal time changes (UK/Europe), see DST page.

T

• Temporal Hours: Historical system dividing daylight into equal hours, varying with seasons.
• Terrestrial Time (TT): Time standard for astronomical calculations, based on TAI.
• Time Dilation (relativistic concept): Time slowing in high-speed or gravitational fields, predicted by Einstein’s relativity (context in FAQ).
• Time Difference: Difference in local times between two locations, important for travel planning; see time zones and map.
• Time Offset: Difference between local time and UTC, e.g., UTC+03:00; see offsets table.
• Time Signal: Broadcast signal for clock synchronization, e.g., BBC time pips.
• Time Standard: Reference system for measuring time, e.g., UTC.
• Time Synchronization: Aligning clocks to a common reference (e.g., NTP), vital for banking and communication.
• Time Zone: Region with a uniform standard time; see Time Zones and the Time Zone Map.
• Time Zone Abbreviation: Short code for time zones, e.g., EST, CET (see examples in FAQ).
• Time Zone Boundary: Geographical line separating time zones, sometimes following country borders (see map).
• Time Zone Conversion: Calculating time differences between zones, used for scheduling meetings (tools in FAQ).
• Time Zone Database (tzdb): Database of time zone rules (e.g., IANA tz) used in software; related to time zone changes.
• Timekeeping: Practice of measuring and recording time, from sundials to atomic clocks.
• Timekeeping Authority: Organization responsible for official time standards (e.g., NIST), often referenced by UTC.
• Timekeeping Error: Inaccuracy in a clock’s time measurement, may accumulate over time.
• Time Lag: Delay in time synchronization or signal transmission, relevant in satellites and communication.

U

• Universal Coordinated Time (UTC): Synonym for Coordinated Universal Time, global civil standard.
• Universal Time (UT): Time standard based on Earth’s rotation; see relation to UTC in FAQ.
• Universal Time 1 (UT1): UT adjusted for Earth’s irregular rotation, used in astronomy.
• Universal Time 2 (UT2): UT1 with seasonal variation corrections (rarely used).
• UTC Offset: Difference in hours/minutes from UTC, e.g., UTC+02:00; see Standard Time Offsets.

W

• World Clock: Device or system displaying times for multiple locations (explore world capitals for quick reference).
• World Time: Collective term for the global time zone system, integrating all local times (overview at Time Zones).

Z

• Z Time (Zulu Time): Military/aviation term for UTC, used in flight planning.
• Zone Time: Time used in a specific time zone, often nautical or legal definition; see zones.

This glossary is part of WorldwideClock.com’s mission to make global time easy to understand. By exploring these definitions, you gain insight into the complexities of international time zones, daylight saving rules, and the science of accurate timekeeping (why Earth’s rotation subtly changes: see details). Whether you want to check the current local time in major world capitals, plan meetings across regions using UTC, or browse the standard time offsets, this resource will guide you. Keep this page bookmarked as a practical reference whenever you need clarity about time zones, UTC offsets, or daylight saving time. For further help or suggestions, visit our FAQ or Contact page.