>The mod command gives stange results when done this way:
>Mod(-13,4). I would expect this to return either 1 or -1 but it returns 3.
>Visual Basic return -1 for this command but Ruby return the same thing as VFP. Can someone explain this to me.... Thanks.

It's a definition by computer languages and in many languages including VFP remainder R is >= 0 and < = n2.
Mod(n1, n2) = n1 - n2[n1/n2] && where [n1/n2] is a floor function
where result should be 3.

Definition could be questioned and further VFP's implementation (ie: why would it return a negative value if it was mod(13,-4) but knowing that for your particular language you could work it.

I'm not an expert in arithmetic definitions:) Just my .02 cents.
Cetin