What's meaning of the Number Format?

[xiao]

All,
I tried to get the number format of each cell. One example is that if I set
Date="*Wednesday, March 14, 2001" in the Format Cells Dialog, the number
format string I got is "[$-F800]dddd, mmmm dd, yyyy".
Anyone knows the meaning of "[$-F800]" in this format?

Also, I got "[$-409]d-mmm;@" in another date format.
What is the meaning of ""[$-409]"?
Thx!