The Truncated Julian Day is a parallel grouped binary time code designed for spacecraft applications. Introduced by NASA and Goddard in 1979, it is a four-digit day count from MJD 40000, or May 24, 1968 expressed as a binary number. This binary code is a derivative of the Julian Day calendar, used by astronomers to count days. All days are consecutively numbered from Julian Day 0, or January 1, 4713 B.C. Only whole solar days are represented under Julian systems.
At noon Greenwich Mean Time, the standard time of London, England, the Julian day begins. The Gregorian calendar, the standard modern calendar system as of 2015, was introduced by Pope Gregory XIII in 1582. Prior to this time, the Julian calendar, as established by Julius Caesar about 50 B.C., was the standard. The units the Gregorian calendar system uses are not constant, for example the 366-day leap year, while the Julian Calendar is constant.
A Julian Period is an interval of 7980 chronological years from 4713 B.C. The next Julian Period begins in 3268 A.D. Previously limited to four digits, today the TJD field is 16 bits, ever since the introduction of PB-5J. This allows for values up to 65535, and records five digits after TJD 9999.