- Ordinal Dates
- Decimal Years
Julian Dates are the whole Julian day number plus the fractional day. (These should not be confused with ordinal dates, which are sometimes erroneously called "Julian dates," or with Julian calendar dates.) They are used to calculate the exact time between any two events in history, such as variable star observations. The decimal fraction for Julian Dates is 0.0 at noon GMT and 0.5 at midnight. To to be precise, modern Julian Dates are usually given according to Coordinated Universal Time (UTC, which is kept within a fraction of a second of GMT by adding leap seconds) or Terrestrial Time (TT, which is not adjusted with leap seconds, and is currently about a half-minute ahead of UTC).
The Truncated Julian Day was introduced by NASA in 1979 as part of a parallel grouped binary time code (PB-5) "designed specifically, although not exclusively, for spacecraft applications." TJD was a 4-digit day count from MJD 44000, which was May 24, 1968, represented as a 14-bit binary number. Since this code was limited to four digits, TJD recycled to zero on MJD 45000, or October 10, 1995, "which gives a long ambiguity period of 27.4 years". (NASA codes PB-1—PB-4 used a 3-digit day-of-year count.) Only whole days are represented. Time of day is expressed by a count of seconds of a day, plus optional milliseconds, microseconds and nanoseconds in separate fields. Later PB-5J was introduced which increased the TJD field to 16 bits, allowing values up to 65535, which occurs in the year 2147. There are five-digits recorded for TJDs after 1995.
- The Reduced Julian Date truncates the first two digits of the Julian Date, but unlike the Modified Julian Date, it keeps the astronomical day beginning at noon UT.
- The Dublin Julian Date was named for the 1955 meeting of the International Astronomical Union in Dublin, Ireland and counts days from the 1900 epoch, which was 1899 December 31 at noon GMT.
- The Heliocentric Julian Date is the same as the (geocentric) Julian Date, except that it is corrected so that the frame of reference is the sun or, more precisely, the center of gravity of the solar system, and thus may be up to 16 minutes different from the Geocentric Julian Date.
- The Chronological Julian Date was proposed recently by Peter Meyer to be the same as the (astronomical) Julian Date, but instead of beginning the day at noon GMT, it starts at midnight relative to whatever timezone is being referred to.
Microsoft Excel is a spreadsheet program which uses a "serial value" count of days to store both dates and time of day. Excel supports two different epochs. The standard epoch, inherited from Lotus 1-2-3 and used by default in Excel for Windows PC, uses January 1, 1900, as Day 1. Excel for Macintosh uses the epoch of the Apple Macintosh's clock, January 1, 1904, as Day 0. Either epoch may be selected in the program options. Days always start at midnight, local civil time, including when Daylight Saving Time is in effect.
(By error Lotus calculated 1900 as a leap year, even though in the Gregorian calendar it is not. Due to this, day 60 converts to February 29, 1900, which did not exist. Some software converts day 1 to December 31, 1899, instead.)
Functions within the program convert time/date strings to serial values. DATEVALUE() takes any date or date/time string and returns the serial day integer, and TIMEVALUE() takes a time or date/time string and returns the fractional day. Functions YEAR(), MONTH(), DAY(), WEEKDAY(), HOUR(), MINUTE() and SECOND() take a serial value as an argument and return the specified date/time element. NOW() returns the current serial date, including both the integral and fractional parts, while TODAY() returns only the serial day integer. Cells containing serial dates may be viewed as either a serial number, such as 38251.924194, or as a standard date/time string by changing the cell format.
ISO 8601 as the year number combined with the ordinal day of the year, i.e. the number of days from the beginning of the current year. The day of year is often erroneously called the "Julian date". The day of the year is numbered 001-365 (366 in leap years).
NASA/NORAD's Two-Line Elements for the tracking of orbital satellites include an "epoch date" which includes a two-digit epoch year and the ordinal day of year, including the fractional day with eight decimal places of precision, in Universal Time. Years are in the range 1957-2056. For example, 99365.99999999 was immediately followed by 00001.00000000, representing the transition from 1999-12-31 to 2000-01-01.
Astronomers sometimes use dates comprising of the year and the decimal fraction of the year. Two slightly different standards have been used, differing by about one half-day:
The stardates in the TV program Star Trek: The Next Generation (TNG) divided the year into 1000 stardates. It was never stated whether stardate years started with the calendar year. Stardate 41986.0 was set in the year 2364, which would make the epoch 41.986 years earlier, either sometime in the year 2322 or the beginning of 2323. There was never a detailed standard published for converting dates, so fans have developed various versions. The game Star Trek Online converts current dates into TNG stardates 400 years in the future using an epoch of May 25, 1922 (2322-400).