I think "broken" is too harsh a word. I see why this was done, so that this number can be used an index into an array of text translation values. A lot of stuff in the C library is truly ancient and predates C. Nowadays, I would imagine that location "0" would just get "burned" and not used so that the indicies have the normal human meaning. But an ancient decision made another choice.
my @monthText = qw (jan feb mar apr may jun jul aug sep oct nov dec);
print "$monthText[0] is for January\n";
print "$monthText[11] is for December\n";