Date fun

[Dennis Boone]

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: