A much more insidious problem with the Y2K bug was the leap year calculation. As you point out, the 20-digit-year thing was relatively easy to fix.
It taught me a great lesson about results not proving correctness as how you got there could bite you later.
> This method works fine for the year 2000 (because it is a leap year), and will not become a problem until 2100, when older legacy programs will likely have long since been replaced.