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Decimal Time - Glossary
Below are abbreviations and definitions of terms used on this site
Analog clock A traditional clock with round face and continually moving hands pointing to numerals or tick-marks
Anno Domini From Latin for "Lord's Year," supposedly a count of years from Jesus Christ's birth, although actually several years in error, abbreviated A.D.
Apparent solar time Time determined by the actual apparent motion of the sun in the sky, as displayed on a sundial, which can different by several minutes from mean solar time
Astronomical Time Time reckoned with the day starting at noon
Atomic time See International Atomic Time
Autumnal equinox The instant in the year when the earth's axis is perpendicular to the sun while days get shorter in the northern hemisphere, marking the start of autumn or fall.
Biel Mean Time According to Swatch, mean solar time in Biel, Switzerland; actually, Central European Time, about one-half hour ahead of mean solar time in Biel
Bissextile Relating to leap years or leap days, from Latin bis sextum, "second sixth [day of kalends of March]"
Centiday 0.01 (1/100) of a day
Century One hundred years, or one hundred of anything
Christian Era Another name for anno Domini
Chronological Julian Date Variation of Julian Date invented by Peter Meyer in which the chronological Julian Day starts at midnight local time (GMT if not specified) or whenever days start on the calendar being referenced
Common Era Another name for anno Domini that does not reference religion, abbreviated CE
Coordinated Universal Time The civil version of Universal Time, kept within one second of UT1 (Greenwich Mean Time) by adding (or subtracting) whole atomic seconds whenever the earth's rotation cause these to diverge
Date Identification of a particular calendar day, expressed by some combination of the year, month, week, and/or day
Day Most commonly, the time it takes the earth to rotate once in relation to the sun, on average 86400 seconds
Daylight Saving Time Practice in some countries of shifting the clocks one hour ahead, usually during the summer, thus extending daylight in the evening and saving energy on light, in some countries called "Summer Time"
Decade Ten years, or ten of anything
Décade In the French Republican Calendar, a period of 10 days meant to replace the 7-day week, also being exactly one-third of a Republican month
Deciday 0.1 (1/10) of a day
Decimal day Same as fractional day
Decimal fraction The fractional part of a decimal number, represented by the digits to the right of the decimal point or sign, equivalent to the whole value of these digits divided by some power of ten
Decimal hour 1/10 of a day in French Decimal Time
Decimal hour decimal fraction of a standard hour (usually 1/10s or 1/100s) used in time clocks and accounting systems
Decimal minute 1/100 of a decimal hour (1/1000 of a day) in French Decimal Time
Decimal number Base-10 representation, where the position of each digit represents a power of ten
Decimal second 1/100 of a decimal minute (1/100000 of a day) in French Decimal Time
Decimal sign The symbol separating the integral and fractional parts of a decimal number, usally a decimal point (dot, period or full stop) in most English-speaking countries, and as a comma in many other countries
Decimal time Any system of time in which units have decimal relationships with other units
Decimalized Universal Time Fractional days when reckoned in Universal Time
Digital clock Modern clock, usually electronic, displaying discrete numerical digits
Epoch The starting point of any system of time
Epoch Day Ordinal day of the year with fractional day, used to track satellites, usually follows the epoch year in an epoch date
Equinox Either of two instants in the year when the earth's axis is perpendicular to the sun, and the nights are roughly equal to the days, marking the beginning of spring or autumn
Fractional day Time of the day represented as a decimal fraction, which can be added to calendar, ordinal or integer dates, found by dividing the number of clock hours by 24
French Republican Calendar Calendar introduced in the French Republic in 1793 and abolished in 1805, which had 12 months, each month containing 3 weeks of 10 days each, followed by 5 or 6 intercalary days, beginning each year on the autumnal equinox
French Revolutionary Time Decimal time system introduced with the French Republican Calendar, in which the day was divided into ten hours (1-10), each hour into 100 decimal minute and each minute into 100 decimal seconds. It was little used and officially abandonded after two years
Greenwich Mean Astronomical Time Time reckoned with the day starting at noon in Greenwich
Greenwich Mean Time Mean solar time measured at the observatory in Greenwich, England, which became the basis for the Prime Meridian and Standard Time; abbreviated GMT, it has been replaced as the international standard by Universal Time, but is often used informally as a synonym
Gregorian calendar Reform of the Julian calendar, introduced in 1582 CE by Pope Gregory, differing in that three leap years are skipped every four centuries, each calendar year being on average 365.2425 days
Integer date Any dating system which counts whole days from a fixed epoch, such as Julian Days
International Atomic time Time determined from very precise atomic clocks, for which one day is defined as exactly 86400 SI seconds, regardless of the actual rotation of the earth, also called International Atomic Time (TAI); differs from UTC by 32 seconds in 2004
International Date Line Imaginary zig-zag line through the Pacific Ocean which separates two consecutive calendar dates, countries immediately east of the line being one calendar day behind countries immediately to the west, abbreviated IDL; IDLE refers to the time zone at the IDL 12 hours east of UT, and IDLW 12 hours west of UT
International System of Units The modern international metric standard, abbreviated SI
ISO 8601 International standard for representation of dates and times, mandates that Gregorian calendar dates be represented in the form YYYY-MM-DD, amongst other things
Julian calendar Reform of the Roman calendar, introduced in 46 BCE by Julius Caesar, differing in that each year had exactly 365 days, except that every fourth year had 366 days, being on average 365.25 days; currently dates are 13 days behind Gregorian
Julian century Exactly 100 Julian years, or 36525 days
Julian Date Julian Day number combined with the fractional day, .00000 being reckoned as noon UT, used by astronomers to date star observations; sometimes also used to refer to ordinal dates, or dates on the Julian calendar
Julian Date time The fractional part of the Julian Date, being the fractional day since noon UT
Julian Day The number of days since the beginning of the Julian Period for a given date, the day being reckoned as starting at noon UT, introduced by the astronomer John Herschel
Julian Period Period of 7980 years, being the length of time before three chronological cycles repeat; the current Julian Period began in the year 4713 BCE
Julian year Period of exactly 365.25 days, used to determine epochs for celestial coordinates
Leap day Extra day added to the calendar to synchronize it with the seasons, 24 February in the Julian and Gregorian calendars (now usually considered 29 February)
Leap second Extra second added to the day to keep clocks synchronized to the motion of the earth, usually added every 8 to 10 years, whenever the difference approaches a whole second, as 23:59:60 on 30 June or 31 December
Leap year Calendar years in which a leap day is added, every four years in the Julian calendar, almost every four in the Gregorian
Mean solar day Average of all the apparent solar days throughout the year as displayed on clocks, very equal to 86400 atomic seconds
Metric system Coordinated system of units originally based upon the metre, introduced by the French in the 1790s, as defined by a number of different standards since then, the modern standard being called SI
Metric time A measure of time interval which uses a single base unit, with multiple and submultiple units derived by adding decimal metric prefixes; represented by the SI second in the modern metric system
Microday 0.000 001 (1/1,000,000) day
Milliday 0.001 (1/1000) day
Millennium One thousand years
Modified Julian Date A shortened form of Julian Date, with the first two digits truncated, and the days beginning at midnight UT instead of noon UT, used to track artificial space satellites, defined as being the Julian Date - 2400000.5
Month In most solar calendars, 1/12 of a year, being from 28 to 31 days long; in lunar calendars, a period corresponding to the time between two new moons, averaging 29.53 days
Ordinal date The year combined with the day of the year
Prime Meridian Imaginary line from pole to pole, passing through the Royal Observatory at Greenwich, England, used as the point of origin (zero) for longitudes, upon which Greenwich Mean Time, and later Universal Time, was based
Roman calendar Calendar used by the Roman Republic before Julius Caesar, which attempted to reconcile lunar months and solar years through the addition of intercalary periods, which became corrupted for political purposes
Second As a unit of time, originally 1/86400 day or 1/60 minute, from "second minute" or pars minuta secunda (2nd small part) now usually referring to the SI or atomic second, defined as being the time it takes a cesium-133 isotope to vibrate 9,192,631,770 times
Serial date Any dating system which counts whole and fractional days from a fixed epoch, such as Julian Dates
Serial day number Whole part of a serial date
Solar time Time as reckoned by the apparent or mean motion of the sun
Standard Time International system adopted in 1884 which recognized GMT (now UTC) as the base time, and divided the rest of the world into time zones at one-hour intervals
Stardates Fictional system of time used on the television series Star Trek and its spin-offs
Summer Time See "Daylight Saving Time"
Swatch Internet Time Decimal time introduced by the Swiss watch company, in which fractional days are displayed as a count of ".beats," each beat equalling one milliday, preceded by the symbol "@" and beginning each day at midnight local standard time in Switzerland
Time interval Measurement of a duration of time from any starting point, such as "I worked 8 hours (or 0.333 day) today"
Time of day Time interval from the beginning of the day, such as "I started work at 8 o'clock (or 08:00 or 8 AM or .333) today"
Time Zone Region where local time is offset from Universal Time by a given amount, usually an even number of hours
Tropical year Also called "solar year," the length of time it takes the earth to reach the same axial orientation relative to the sun, the exact value depending upon which orientation is being referred to, the mean value for all orientations being about 365.24219 days
Truncated Julian Date A shortened form of Julian Date used by NASA, originally with the first three digits truncated, and the days beginning at midnight UT instead of noon UT, officially defined as the Julian Date - 2440000.5
Unix Time A count of seconds since the beginning of 1970, used for dates and times on Unix computers and other computer systems
Universal Time The world-wide standard time of the day upon which most local times are based, based upon Greenwich Mean Time, begins each day at mean midnight on the Prime Meridian, includes several versions depending upon corrections for the earth's motion
Universal Time, Coordinated See Coordinated Universal Time
Vernal equinox The instant in the year when the earth's axis is perpendicular to the sun while days get longer in the northern hemisphere, marking the start of spring, the average time between successive vernal equinoxes currently being about 365.242374 days
Week Repeating cycle of seven days, used to determine religious observances and work schedules
Week date Dates expressed by ordinal week of the year and ordinal day of week, as defined by ISO 8601
Year One revolution of the earth around the sun, approximately 365 days, the exact length depending upon the type of year being referred to
Zulu Time Alternate name for Universal Time, from the military phonetic alphabet, "zulu" representing "zero," referring to Universal Time being the time at zero longitude
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AD Anno domini
AE Autumnal equinox
BC Before Christ
BCE Before the Common Era
BMT Biel Mean Time
cd Centiday (unofficial)
CE Common Era
d Day (official)
dd Deciday (unofficial)
DST Daylight Saving Time
ER Era of the (French) Republic
FRC French Republican Calendar
FRT French Revolutionary Time
GMT Greenwich Mean Time
JD Julian Date
JDN Julian Day Number
IDL International Date Line
IDLE International Date Line - East
IDLW International Date Line - West
ISO International Organization for Standardization
h Hour
md Milliday (unofficial)
μd Microday (unofficial)
min Minute
MJD Modified Julian Date
MJDN Modified Julian Day Number
M.S.D. Mean solar day
s second
SI International System of Units (Le Système International d'Unités in French)
TAI International Atomic Time
TJD Truncated Julian Date
UT Universal Time
UTC Universal Time, Coordinated
VE Vernal equinox
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Definitions
Ab Urbe Condita From Latin "from the founding of the city," era of the Roman calendar using the mythical founding of the city of Rome in 753 BCE as its epochAnalog clock A traditional clock with round face and continually moving hands pointing to numerals or tick-marks
Anno Domini From Latin for "Lord's Year," supposedly a count of years from Jesus Christ's birth, although actually several years in error, abbreviated A.D.
Apparent solar time Time determined by the actual apparent motion of the sun in the sky, as displayed on a sundial, which can different by several minutes from mean solar time
Astronomical Time Time reckoned with the day starting at noon
Atomic time See International Atomic Time
Autumnal equinox The instant in the year when the earth's axis is perpendicular to the sun while days get shorter in the northern hemisphere, marking the start of autumn or fall.
Biel Mean Time According to Swatch, mean solar time in Biel, Switzerland; actually, Central European Time, about one-half hour ahead of mean solar time in Biel
Bissextile Relating to leap years or leap days, from Latin bis sextum, "second sixth [day of kalends of March]"
Centiday 0.01 (1/100) of a day
Century One hundred years, or one hundred of anything
Christian Era Another name for anno Domini
Chronological Julian Date Variation of Julian Date invented by Peter Meyer in which the chronological Julian Day starts at midnight local time (GMT if not specified) or whenever days start on the calendar being referenced
Common Era Another name for anno Domini that does not reference religion, abbreviated CE
Coordinated Universal Time The civil version of Universal Time, kept within one second of UT1 (Greenwich Mean Time) by adding (or subtracting) whole atomic seconds whenever the earth's rotation cause these to diverge
Date Identification of a particular calendar day, expressed by some combination of the year, month, week, and/or day
Day Most commonly, the time it takes the earth to rotate once in relation to the sun, on average 86400 seconds
Daylight Saving Time Practice in some countries of shifting the clocks one hour ahead, usually during the summer, thus extending daylight in the evening and saving energy on light, in some countries called "Summer Time"
Decade Ten years, or ten of anything
Décade In the French Republican Calendar, a period of 10 days meant to replace the 7-day week, also being exactly one-third of a Republican month
Deciday 0.1 (1/10) of a day
Decimal day Same as fractional day
Decimal fraction The fractional part of a decimal number, represented by the digits to the right of the decimal point or sign, equivalent to the whole value of these digits divided by some power of ten
Decimal hour 1/10 of a day in French Decimal Time
Decimal hour decimal fraction of a standard hour (usually 1/10s or 1/100s) used in time clocks and accounting systems
Decimal minute 1/100 of a decimal hour (1/1000 of a day) in French Decimal Time
Decimal number Base-10 representation, where the position of each digit represents a power of ten
Decimal second 1/100 of a decimal minute (1/100000 of a day) in French Decimal Time
Decimal sign The symbol separating the integral and fractional parts of a decimal number, usally a decimal point (dot, period or full stop) in most English-speaking countries, and as a comma in many other countries
Decimal time Any system of time in which units have decimal relationships with other units
Decimalized Universal Time Fractional days when reckoned in Universal Time
Digital clock Modern clock, usually electronic, displaying discrete numerical digits
Epoch The starting point of any system of time
Epoch Day Ordinal day of the year with fractional day, used to track satellites, usually follows the epoch year in an epoch date
Equinox Either of two instants in the year when the earth's axis is perpendicular to the sun, and the nights are roughly equal to the days, marking the beginning of spring or autumn
Fractional day Time of the day represented as a decimal fraction, which can be added to calendar, ordinal or integer dates, found by dividing the number of clock hours by 24
French Republican Calendar Calendar introduced in the French Republic in 1793 and abolished in 1805, which had 12 months, each month containing 3 weeks of 10 days each, followed by 5 or 6 intercalary days, beginning each year on the autumnal equinox
French Revolutionary Time Decimal time system introduced with the French Republican Calendar, in which the day was divided into ten hours (1-10), each hour into 100 decimal minute and each minute into 100 decimal seconds. It was little used and officially abandonded after two years
Greenwich Mean Astronomical Time Time reckoned with the day starting at noon in Greenwich
Greenwich Mean Time Mean solar time measured at the observatory in Greenwich, England, which became the basis for the Prime Meridian and Standard Time; abbreviated GMT, it has been replaced as the international standard by Universal Time, but is often used informally as a synonym
Gregorian calendar Reform of the Julian calendar, introduced in 1582 CE by Pope Gregory, differing in that three leap years are skipped every four centuries, each calendar year being on average 365.2425 days
Integer date Any dating system which counts whole days from a fixed epoch, such as Julian Days
International Atomic time Time determined from very precise atomic clocks, for which one day is defined as exactly 86400 SI seconds, regardless of the actual rotation of the earth, also called International Atomic Time (TAI); differs from UTC by 32 seconds in 2004
International Date Line Imaginary zig-zag line through the Pacific Ocean which separates two consecutive calendar dates, countries immediately east of the line being one calendar day behind countries immediately to the west, abbreviated IDL; IDLE refers to the time zone at the IDL 12 hours east of UT, and IDLW 12 hours west of UT
International System of Units The modern international metric standard, abbreviated SI
ISO 8601 International standard for representation of dates and times, mandates that Gregorian calendar dates be represented in the form YYYY-MM-DD, amongst other things
Julian calendar Reform of the Roman calendar, introduced in 46 BCE by Julius Caesar, differing in that each year had exactly 365 days, except that every fourth year had 366 days, being on average 365.25 days; currently dates are 13 days behind Gregorian
Julian century Exactly 100 Julian years, or 36525 days
Julian Date Julian Day number combined with the fractional day, .00000 being reckoned as noon UT, used by astronomers to date star observations; sometimes also used to refer to ordinal dates, or dates on the Julian calendar
Julian Date time The fractional part of the Julian Date, being the fractional day since noon UT
Julian Day The number of days since the beginning of the Julian Period for a given date, the day being reckoned as starting at noon UT, introduced by the astronomer John Herschel
Julian Period Period of 7980 years, being the length of time before three chronological cycles repeat; the current Julian Period began in the year 4713 BCE
Julian year Period of exactly 365.25 days, used to determine epochs for celestial coordinates
Leap day Extra day added to the calendar to synchronize it with the seasons, 24 February in the Julian and Gregorian calendars (now usually considered 29 February)
Leap second Extra second added to the day to keep clocks synchronized to the motion of the earth, usually added every 8 to 10 years, whenever the difference approaches a whole second, as 23:59:60 on 30 June or 31 December
Leap year Calendar years in which a leap day is added, every four years in the Julian calendar, almost every four in the Gregorian
Mean solar day Average of all the apparent solar days throughout the year as displayed on clocks, very equal to 86400 atomic seconds
Metric system Coordinated system of units originally based upon the metre, introduced by the French in the 1790s, as defined by a number of different standards since then, the modern standard being called SI
Metric time A measure of time interval which uses a single base unit, with multiple and submultiple units derived by adding decimal metric prefixes; represented by the SI second in the modern metric system
Microday 0.000 001 (1/1,000,000) day
Milliday 0.001 (1/1000) day
Millennium One thousand years
Modified Julian Date A shortened form of Julian Date, with the first two digits truncated, and the days beginning at midnight UT instead of noon UT, used to track artificial space satellites, defined as being the Julian Date - 2400000.5
Month In most solar calendars, 1/12 of a year, being from 28 to 31 days long; in lunar calendars, a period corresponding to the time between two new moons, averaging 29.53 days
Ordinal date The year combined with the day of the year
Prime Meridian Imaginary line from pole to pole, passing through the Royal Observatory at Greenwich, England, used as the point of origin (zero) for longitudes, upon which Greenwich Mean Time, and later Universal Time, was based
Roman calendar Calendar used by the Roman Republic before Julius Caesar, which attempted to reconcile lunar months and solar years through the addition of intercalary periods, which became corrupted for political purposes
Second As a unit of time, originally 1/86400 day or 1/60 minute, from "second minute" or pars minuta secunda (2nd small part) now usually referring to the SI or atomic second, defined as being the time it takes a cesium-133 isotope to vibrate 9,192,631,770 times
Serial date Any dating system which counts whole and fractional days from a fixed epoch, such as Julian Dates
Serial day number Whole part of a serial date
Solar time Time as reckoned by the apparent or mean motion of the sun
Standard Time International system adopted in 1884 which recognized GMT (now UTC) as the base time, and divided the rest of the world into time zones at one-hour intervals
Stardates Fictional system of time used on the television series Star Trek and its spin-offs
Summer Time See "Daylight Saving Time"
Swatch Internet Time Decimal time introduced by the Swiss watch company, in which fractional days are displayed as a count of ".beats," each beat equalling one milliday, preceded by the symbol "@" and beginning each day at midnight local standard time in Switzerland
Time interval Measurement of a duration of time from any starting point, such as "I worked 8 hours (or 0.333 day) today"
Time of day Time interval from the beginning of the day, such as "I started work at 8 o'clock (or 08:00 or 8 AM or .333) today"
Time Zone Region where local time is offset from Universal Time by a given amount, usually an even number of hours
Tropical year Also called "solar year," the length of time it takes the earth to reach the same axial orientation relative to the sun, the exact value depending upon which orientation is being referred to, the mean value for all orientations being about 365.24219 days
Truncated Julian Date A shortened form of Julian Date used by NASA, originally with the first three digits truncated, and the days beginning at midnight UT instead of noon UT, officially defined as the Julian Date - 2440000.5
Unix Time A count of seconds since the beginning of 1970, used for dates and times on Unix computers and other computer systems
Universal Time The world-wide standard time of the day upon which most local times are based, based upon Greenwich Mean Time, begins each day at mean midnight on the Prime Meridian, includes several versions depending upon corrections for the earth's motion
Universal Time, Coordinated See Coordinated Universal Time
Vernal equinox The instant in the year when the earth's axis is perpendicular to the sun while days get longer in the northern hemisphere, marking the start of spring, the average time between successive vernal equinoxes currently being about 365.242374 days
Week Repeating cycle of seven days, used to determine religious observances and work schedules
Week date Dates expressed by ordinal week of the year and ordinal day of week, as defined by ISO 8601
Year One revolution of the earth around the sun, approximately 365 days, the exact length depending upon the type of year being referred to
Zulu Time Alternate name for Universal Time, from the military phonetic alphabet, "zulu" representing "zero," referring to Universal Time being the time at zero longitude
Abbreviations
AUC Ab Urbe ConditaAD Anno domini
AE Autumnal equinox
BC Before Christ
BCE Before the Common Era
BMT Biel Mean Time
cd Centiday (unofficial)
CE Common Era
d Day (official)
dd Deciday (unofficial)
DST Daylight Saving Time
ER Era of the (French) Republic
FRC French Republican Calendar
FRT French Revolutionary Time
GMT Greenwich Mean Time
JD Julian Date
JDN Julian Day Number
IDL International Date Line
IDLE International Date Line - East
IDLW International Date Line - West
ISO International Organization for Standardization
h Hour
md Milliday (unofficial)
μd Microday (unofficial)
min Minute
MJD Modified Julian Date
MJDN Modified Julian Day Number
M.S.D. Mean solar day
s second
SI International System of Units (Le Système International d'Unités in French)
TAI International Atomic Time
TJD Truncated Julian Date
UT Universal Time
UTC Universal Time, Coordinated
VE Vernal equinox