Just saying, a little clarification would have helped. The details are there, but putting it together would have laid out the base algorithm that's being optimized, which I see as a basic point of an article like this, give everyone a clear understanding of the algorithm so they can follow along.

"rd is number of days offset from 1970-01-01, positive or negative. In some languages the mod 7 can yield a negative in the range -6 to 0, so an extra +7 and mod is used in the basic implementation."