What is the precise definition of JDE’s Julian Date format?

I am writing code to convert from a Gregorian date to a JDE (J.D.Edwards) Julian date.

Note: a JDE Julian date is different from the normal usage of the term Julian date.

As far as I can work out from Googling, the definition of a JDE Julian date is:

1000*(year-1900) + dayofyear

where year is the 4-digit year (e.g. 2009), and dayofyear is 1 for 1st January, and counts up all year to either 365 or 366 for 31st December (depending whether this is a leap year).

My question is this: are years before 1900 supported? If so, does the above formula still hold, or should it be this:

1000*(year-1900) - dayofyear

(note minus instead of plus.)

or something else?

Does anyone have a link to the official documentation for this date format?

What is the precise definition of JDE’s Julian Date format?