Actually there are trivially easy ways to calculate DOW off the top of your head. No one bothers because it was never a real problem. People used to carry their

Actually there are trivially easy ways to calculate DOW off the top of your head. No one bothers because it was never a real problem. People used to carry their calendar books on them to keep track of their engagements. Now we carry a smartphone. Nevertheless, my colleague bothered to do it and he could tell you the DOW without pausing. He is of average intelligence maybe a little more intelligent than I (though I was better at mental math)