Do you remember the Y2K problem that worried everyone in late 1999? The problem was that a large number of dates, in all kinds of systems, listed 2-digit years, such as 85 instead of 1985. How would systems know whether 05 means 1905 or 2005? Fortunately, the predicted disaster didn’t happen, but there is still a Y2K problem because of the ambiguity it can create. Excel handles it (at least for now) by interpreting a 2-digit year less than 30 as being in the 21st century, and all others as being in the 20th century. For example, it interprets 3/14/17 as March 14, 2017 and 5/20/72 as May 20, 1972. Of course, the easiest way to avoid the ambiguity is to use 4-digit years whenever you enter dates. This is the main lesson of Y2K!
Try it! In the spreadsheet above, enter a date such as 3/14/29 (the last two digits less than 30). Then enter another date such as 5/20/30 (the last two digits greater than or equal to 30). Watch how Excel transforms the year automatically. Do you see how this could be a problem if you really meant 3/14/29 to be the birthday of a person born in 1929?
