At 7/3/2003 04:47 PM (Thursday), Martijn Tonies wrote:

> > The first longword is the number of days since 17 November 1858.
> > (That's a long story).
>
>Care to do a write up and add it to the History part at cvalde.com?
>
>I'm curious :)

I believe Interbase used this date because the VAX/VMS system used it. I
believe DEC chose it because they had to choose something and it was
already in use. (I think one key market for VAX/VMS was the systems market
dominated by Control Data Corp for computers which would conveniently
handle very large numbers. Their machines had 48 and 60 bit words. I know
the Smithsonian Astronomical Observatory was one of the first users of
VAX/VMS systems and replaced one or more CDC machines with one or more
VAX/VMS machines.)

From http://www.iwpcug.org/docs/18991230.htm :

Astronomers, for example, use 17 November 1858 as day zero for a date known
as MJD = Modified Julian Date. This is actually a modification in smaller
numbers of another day number system called the Julian Date which has a
zero at noon on 1 January 4713 BC of the Julian Proleptic Calendar. That
date is 2,400,000.5 days prior to 0 hours on 17 November 1858.

The Julian Proleptic Calendar, in case you wondered, is the Julian Calendar
extended backwards in time following the same rules as the Julian Calendar
regarding leap years; i.e. every four years without any exceptions. In this
calendar, there is no year zero so that 1 BC is the year prior to 1 AD.
This has the unfortunate consequence that leap years prior to 1 AD are not
those divisible by four but those years which have a remainder of one when
divided by four. Thus the year 4713 BC in this calendar was a leap year.

The days of the week continue without exception in their cycle of seven
days and if you work it out, 1 January 4713 BC was a Monday. This date was
chosen by an Italian named Joseph Scaliger (1540-1609) as preceding all
astronomical events known at that time so that all Julian Dates are positive.