We would like to welcome Professor Addenhauer to these pages. He can usually be found in the middle of a circle of students listening with bated breath as he explains a complicated mathematical concept. He retired a few years ago, but missed the students so much he teaches a class or two every semester. He carries his trusty slide rule everywhere and frequently amazes his audience with his quick answers to complicated equations. We have persuaded him to give us a little of his time to tell you about important mathematical concepts. Do not worry; he always explains in plain English.
Here is a recent letter from him on the topic of dates in Excel: