What Caused the Y2K Bug?

The cause of the Y2K bug is very simple.  Since the 1960's, the vast majority of computer programs have been written to store dates in a YY-MM-DD format.  Storing only two digits for the year saved considerable amounts of storage space.  Storage used to be incredibly expensive.  Lots of money was saved, for example, by banks, who stored records of thousands of transactions a day.  When these programs were written, programmers did not worry about what would happen when year 2000 rolled around, because the assumption was that all the systems would be antiquated by then, and newer systems would have taken over.  These systems were never quite gotten rid of however, and in fact the newer systems were built to be compatible with the older systems.  The problem compounded itself in this manner.