Steve Dirickson writes:

>         <<I must admit that I'm confused, Steve, particularly so about the
>  offset value of 50,448. Most people use January 1, 1900 as the start of
>  their Julian Date calendar, that being Day 1, although any date can be
used,
>  so long as you're consistent. But if January 1, 1997 is to be 50,449, then
>  counting backwards, the reference date falls in the middle of the year
1858.
>  Why start there?>>
>
>
>  That's why I asked in the first reply if he wanted a "real" Julian or
>  Modified Julian date, as opposed to the day-of-year thing that so many
>  people erroneously call a "Julian" date.
>
>  For any given date/time, there's only one Julian Date; it is referenced
from
>  noon, 1 January 4712BC using the Julian proleptic calendar (the name
"Julian
>  date" has nothing to do with Emperor Julius Caesar, but the "Julian
>  proleptic calendar" is the one that he introduced). There's also exactly
one
>  Modified Julian Date, which is referenced to midnight, November 17, 1858,
>  using the Gregorian calendar. MJD is a standard calculation use in
>  astronomy; in fact, the MJD start-date and calculation are recognized by
the
>  International Astronomical Union and specified in CCIR Recommendation
457-1,
>  "USE OF THE MODIFIED JULIAN DATE BY THE STANDARD FREQUENCY AND TIME-SIGNAL
>  SERVICES". The start date is not negotiable, and calling anything
>  else-specifically the year-representation-plus-day-of-year thing-a "Julian"
>  date is a gross-but popular-error.

Although I must admit that I had never heard of CCIR Recommendation 457-1, I
did find several web pages that discuss the standard obliquely. Two of the
better are:

     http://tycho.usno.navy.mil/mjd.html
     http://www.aiag.org/testproc.html

However, I remain unimpressed, particularly so for business purposes.

While astronomers do have a real and pressing need for a calendaric method to
count the number of days from any point in time to any other, the selection of
midnight, November 17, 1858 (which by the way, was the date I inferred from
your previous notes), has no more relevancy as a starting point than any other
for a "modified Julian Date".

What works well for astronomy doesn't necessarily have much value to a
business calculation, especially so when the powerhouse business software
companies such as Microsoft and AICS Research start their JD calendars at
midnight, December 31, 1899.*

Wirt Atmar

* There's a grain of salt to swallowed in here, somewhere.