# Truncated Julian Day (TJD)

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This equation gives you the Truncated Julian Day (TJD) when you input a Gregorian (standard modern calendar date).

The TJD's starting point epoch is midnight on May 24, 1868.

"The Truncated Julian Date (TJD) as introduced by NASA/Goddard 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 40000, 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 50000, 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 will occur in the year 2147. There are five digits recorded after TJD 9999.."1

# Example

If you input January 1, 2000, this equation will return the Truncated Julian Day:  11544.0

If you input May 24, 1968, this equation will return the Truncated Julian Day: 0.0