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:
I do not usually talk specifically about Excel, but when I heard this issue was all about dates and calculating dates, I thought you might need some basics. You need to know three things about Excel dates, and then you will know it all.
One – Excel believes time started on January 1st in the year 1900. No matter what the philosophers and scientists say, what we can see from historical evidence, and just plain common sense, Excel will not change its mind. Day one is 1/1/1900.
Two – A day equals one.
Three – Hours are fractions of days, so in Excel terms, noon is .5 and 8:00 AM is .33.
Excel has been keeping track of all the days since 1/1/1900 using serial numbers. Today’s serial number is greater than 39000. Excel uses these serial numbers to do math with dates. Excel sees a date not as we do, but as its serial number. It generally formats them conveniently for us, but occasionally, you will need to change the format of dates to see the answer you expect.
Let me give you an example: subtract Christmas (12/25/2008) from New Year’s Day (1/1/2009) and you will get 7 days. Excel might display that as 1/7/1900. It is the same thing, just formatted differently.
So remember that time started on January 1st, 1900, Excel counts every day as one, hours are fractions of days, and it all adds up to NOW().
