For years I have seen two different definitions of Truncated Julian Day (TJD). One said that it was the Julian Day with the first three digits truncated, so that it was never more than four digits. The other said that it was a count of days midnight of JD 2,440,000, or May 24, 1968 (MJD 40000.0). These two definitions resulted in the same number, until JD 2,450,000, or October 10, 1995 (MJD 50000.0). By the first definition, TJD changed from 9999 to 0, and by the second to 10000. I never knew the whole story, which definition is correct, and how it came to be, until now.
I just discovered the original proposal for TJD, NASA Technical Memorandum 80606, "A Grouped Binary Time Code for Telemetry and Space Applications" by A. R. Chi, published December 1979 by Goddard Space Flight Center. It was defined as part of NASA time code PB-5:
The Day Count System - The four decimal digit day count system is derived from the truncation of the three most significant numbers of the present seven decimal digits of Julian Day Number (JDN), thus the name the Truncated Julian Day (TJD). TJD is arbitrar:ly chosen to begin from 0 at midnight May 24, 1969, (sic) and ends 9999 at midnight October 9, 1995 after which it recycles to zero. The repetition period is 27.4 years. A conversion table from TJD to calendar data in year, month, and day is given in Table II. The four decimal digits of a TJD number are represented by a 14-bit binary number.
By this definition, TJD is always a truncation of the Julian Day, and is never more than four digits, or 9999. Since it is a binary field represented by 14 bits, it could not physically exceed 16383. Furthermore, this is an integer value. The time of day is not represented as a decimal fraction, but by a count of seconds of the day from midnight, as well as optional milliseconds, microseconds and nanoseconds.
But that's not the whole story. A modified time code called PB-5J was later defined with 16 bits for TJD, which allows for integers up to 65535, corresponding to dates up to October 28, 2147. TJD values greater than 9999 can be found for the year 1995 and after, and a NASA web site converts to and from TJD for any positive value.
So there actually are two different definitions of Truncated Julian Day, one in PB-5 and the other in PB-5J, and it appears to me that the latter superseded the former.