The following was posted to this group a couple years ago (and has probably been posted several times). -Ken ==================== old text below ================================= And here's the story from DEC. This came off of DSIN some time ago and I'm not aware of any copyright protections on this article. ------------ 38 Why Is Wednesday November 17, 1858 The Base Time For VAX/VMS? COMPONENT: SYSTEM TIME OP/SYS: VMS, Version 4.n LAST TECHNICAL REVIEW: 06-APR-1988 SOURCE: Customer Support Center/Colorado Springs QUESTION: Why is Wednesday, November 17, 1858 the base time for VAX/VMS? ANSWER: November 17, 1858 is the base of the Modified Julian Day system. The original Julian Day (JD) is used by astronomers and expressed in days since noon January 1, 4713 B.C. This measure of time was introduced by Joseph Scaliger in the 16th century. It is named in honor of his father, Julius Caesar Scaliger (note that this Julian Day is different from the Julian calendar named for the Roman Emperor Julius Caesar!). Why 4713 BC? Scaliger traced three time cycles and found that they were all in the first year of their cyle in 4713 B.C. The three cycles are 15, 19, and 28 years long. By multiplying these three numbers (15 * 19 * 28 = 7980), he was able to represent any date from 4713 B.C. through 3267 A.D. The starting year was before any historical event known to him. In fact, the Jewish calendar marks the start of the world as 3761 B.C. Today his numbering scheme is still used by astronomers to avoid the difficulties of converting the months of different calendars in use during different eras. So why 1858? The Julian Day 2,400,000 just happens to be November 17, 1858. The Modified Julian Day uses the following formula: MJD = JD - 2,400,000.5 The .5 changed when the day starts. Astronomers had considered it more convenient to have their day start at noon so that nighttime observation times fall in the middle. But they changed to conform to the commercial day. 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: 31-JUL-31086 02:48:05.47 At this time, all clocks and time-keeping operations within VMS will suddenly stop, as system time values go negative. Note that all time display and manipulation routines within VMS allow for only 4 digits within the 'YEAR' field. We expect this to be corrected in a future release of VAX/VMS sometime prior to 31-DEC-9999. -- Ken Worsham SAS Institute Inc worsham@vms.sas.com