Programmers wanting to avoid the Y2K bug had two broad options: entirely rewrite their code, or adopt a quick fix called “windowing”, which would treat all dates from 00 to 20, as from the 2000s, rather than the 1900s. An estimated 80 per cent of computers fixed in 1999 used the quicker, cheaper option.

“Windowing, even during Y2K, was the worst of all possible solutions because it kicked the problem down the road,” says Dylan Mulvin at the London School of Economics.

Source: A lazy fix 20 years ago means the Y2K bug is taking down computers now | New Scientist