Operator MOD

[Kernelpanic]

@Chris
Mathematica gives the following answers
Mod[-1, 5] -> 4
Mod[-1.4, 5] -> 3.6
Mod[1.4, 5] -> 1.4
Mod[-7, 5] -> 3
Mod[-7.1, 5] -> 2.9
. . .

Interesting! I've now installed (GNU)Octave and it gives the same results as the expensive Mathematica.

I think I figured out where MOD is used in calculating the calendar. Let's see if my belief is correct.