A brief history of OpenVMS Timekeeping, please?
Why does OpenVMS regards November 17, 1858 as the beginning of time… The modified Julian date adopted by the Smithsonian Astrophysical Observatory (SAO) for satellite tracking is Julian Day 2400000.5, which turns out to be midnight on November 17, 1858. SAO started tracking satellites with an 8K (nonvirtual) 36-bit IBM 704 in 1957 when Sputnik went into orbit. The Julian day was 2435839 on January 1, 1957. This is 11225377 octal, which was too big to fit into an 18-bit field. With only 8K of memory, the 14 bits left over by keeping the Julian date in its own 36-bit word would have been wasted. SAO also needed the fraction of the current day (for which 18 bits gave enough accuracy), so it was decided to keep the number of days in the left 18 bits and the fraction of a day in the right 18 bits of one word. Eighteen bits allows the truncated Julian Day (the SAO day) to grow as large as 262143, which from November 17, 1858, allowed for 7 centuries. Possibly, the date could only grow as l
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