It is not that the clocks reset to 1900. It depends on the clock system used. Some clocks reverted to 1900, others would do strange things like setting the year as 19001. Others would so stranger things, like try to flip the 19 into a 20 and end up with a year of 201900 or 19100. In any case, Missile Clocks were verified with a main clock in some city somewhere. If the Missile Clock did not compare to the main clock correctly, the computer assumed that the main clock is down, the city nuked and it would launch the missiles automatically! The crews manning the silos would be powerless to shut them down. Ain't Mutual Assured Destruction grand?!!
And now all of this a automated, the crews in the silos eliminated.
But on a more easier to the stomach, credit companies a banks also had similar problems and would have shut down everyone's credit and bank accounts, and money would become worthless when Y2K hits. Everyone would become poor all of the sudden! Because of this, there were 2 suicides in Wall Street shortly after Y2K.
Now if we need to get technical, the next Y2K Bug is with the Apple IIs as their Y2K is in 2048 -> 2049. Hopefully the Woz will fix it! (LOL!) There are a few systems that will kick in at 2038 -> 2039, but they are not major. Unix and modern Windows do not have the time on a fixed string anymore, like people do, when it runs out space, it adds another Zero.
