Calculating DOW on a PLC 5

I currently use mostly SLC 5/04 & /05's, these plc's have a DOW function built in (S:53). But I was in need of updating some of our older machines to function like our newer machines. This included a reliable "time switch" to turn platen heaters on and off at specific times (Day, Hour, Minute).

So after some research I discovered Babwani's Congruence which showed a mathematical way to derive the Day from the date information.

I am posting a copy of the PLC 5/20 program and the reference document here. Feel free to comment (I'm sure you will) and to post any enhancements you may have.