MOD function with negative numbers

[roger32909 at bellsouth.net]

If I use the mod function in Excel, say =mod(-1,7), the result is 6.

If I google "-1 modulo 7", the result is 6.

But if I use the PH mod function for MOD(-1,7) the result is -1

$ QUIZ

Q U I Z   (7.10.G1)
Copyright 1998 COGNOS INC. (ALPHA)
Licensed PH-AXP-RUNTIME 

> DEFINE D-1 = MOD(1,7)
> DEFINE D-2 = MOD(2,7)
> DEFINE D-3 = MOD(3,7)
> DEFINE D-4 = MOD(4,7)
> DEFINE D-5 = MOD(5,7)
> DEFINE D-6 = MOD(6,7)
> DEFINE D-7 = MOD(7,7)
> DEFINE D-1N = MOD(-1,7)
>  DEFINE D-2N = MOD(-2,7)
>  DEFINE D-3N = MOD(-3,7)
>  DEFINE D-4N = MOD(-4,7)
>  DEFINE D-5N = MOD(-5,7)
>  DEFINE D-6N = MOD(-6,7)
>  DEFINE D-7N = MOD(-7,7)
> SAVE TEST-MOD
> REPO ALL
> GO
03/01/06                                       System            PAGE   1

    D-1      D-2      D-3      D-4      D-5      D-6      D-7      D-1N

    D-2N     D-3N     D-4N     D-5N     D-6N     D-7N

        1        2        3        4        5        6        0       -1
       -2       -3       -4       -5       -6        0

Are there two different modular arithmetic systems for negative numbers?  Or am I doing something wrong?