It started back in the 1960's when the programmers wanted to save space so they made the year as a two digit number. When the computers' dates turns from "99" to "00", the computer will think it's 1900 instead of 2000. When the computer thinks it is 1900, it will miscalculate the days in the year because 2000 is a leap year and 1900 wasn't.
To add to the problem January 1, 2000 lands on a Saturday so not as many people will be in office that day. As noted by MITRE, instead of the traditional maintenance problem- "Here's the problem, fix it"-the Y2K problem becomes-"Where is the problem, what are the fixes?"
"A problem could occur when two dates are compared, and then the computer reads "00" as 1900; when trying to sort out a bill or subscription and the computer thinks "97", or 1997, is larger than "00", 1900. That means the bill would be considered overdue even when it isn't," said The Denver Post.
Most Y2K problems are found in information systems. Systems everywhere that use dates will be affected by the Y2K problems. Don't think that newer systems are unaffected by Y2K. Only at the end of 1997, a new version of Quicken was released that could handle dates past 1999.