On 2005-08-15 15:04, "Doug McNutt" <[EMAIL PROTECTED]> wrote:

> At 13:31 -0400 8/15/05, Mark Reed wrote:
> If anyone gets serious about Julian dates there is also the Modified Julian
> Date, MJD, used by the US military and others. It differs from the JAD above
> by a large well-defined integer plus 1/2.
The (then) most recent even multiple of 100,000 was chosen for it: MJD 0 =
JD 2,400,000.5.

> The result is a day that begins at
> midnight and starts at a more recent date that I don't remember. It's not Jan
> 0, 1970 though.
November 17, 1858.  Which, while not Jan 1, 1970, is still time zero for
another operating system some of you may have heard of: VMS.

There's also something called the Truncated Julian Day/Date, or TJD, which
NASA used to use: it was essentially the last four digits of the MJD, so
that TJD 0 was MJD 40000, aka May 24, 1968.  But once MJD 50000 rolled
around (on Oct 10, 1995), the TJD became ambiguous.  Besides, while saving a
decimal digit of storage per log entry was significant when NASA was trying
to get computers light enough to launch in the 1960s, it's not exactly an
earth-shattering storage win today.  So the TJD is best avoided.  In fact,
forget I mentioned it. :)

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