Found the following in an old usenet post. Seems on-topic. :)
De
The Modified Julian Day was adopted by the Smithsonian
Astrophysical
Obser- vatory (SAO) in 1957 for satellite tracking. SAO started
tracking satellites with an 8K (non-virtual) 36-bit IBM 704 computer
in 1957, when Sputnik was launched. The Julian day was 2,435,839 on
January 1, 1957. This is 11,225,377 in octal notation, which was too
big to fit into an 18-bit field (half of its standard 36-bit word).
And, with only 8K of memory, no one wanted to waste the 14 bits left
over by keeping the Julian Day in its own 36-bit word. However, they
also needed to track hours and minutes, for which 18 bits gave enough
accuracy. So, they decided to keep the number of days in the left 18
bits and the hours and minutes in the right 18 bits of a word.
Eighteen bits would allow the Modified Julian Day (the
SAO day) to
grow as large as 262,143 ((2 ** 18) - 1). From Nov. 17, 1858, this
allowed for seven centuries. Using only 17 bits, the date could
possibly grow only as large as 131,071, but this still covers 3
centuries, as well as leaving the possibility of representing
negative time. The year 1858 preceded the oldest star catalog in use
at SAO, which also avoided having to use negative time in any of the
satellite tracking calculations.
This base time of Nov. 17, 1858 has since been used by
TOPS-10,
TOPS-20, and VAX/VMS. Given this base date, the 100 nanosecond
granularity implemented within VAX/VMS, and the 63-bit absolute time
representation (the sign bit must be clear), VMS should have no
trouble with time until: