[bbctvtechop]

Of course there was never actually a Year 0. A.D. started with year 1 A.D. of course, which means everyone celebrated the start of the new millennium and the 21st Century exactly one year early. The true and proper beginning of this new millennium and the 21st century was on 1st January 2001. For the case of the Millennium Bug, it was just the fear of computerised systems around the world malfunctioning when the first two digits of the year changed from 19 to 20, and it turned out to be a damp squib anyway!

The Millennium Big worry wasn't that systems with a 4-digit year were changing its first two from 19 to 20.

The problem was with systems which had been coded to use only a 2-digit year (eg 31/12/99) and the worry that when it went to 01/01/00, it would think it had gone backwards by 100 years rather than forwards by one day. 

In other words, does 01/01/00 refer to the year 1900 or the year 2000? There is no way of us humans being able to tell the difference without extra context, so computers stood no chance.

A lot of time, effort and money went into identifying systems which used a 2-digit year and then recoding them to use a 4 digit year.

There is a similar problem to solve before 19th January 2038 - the "epochalypse" - when computers which calculate time in 16-bits will appear to rewind back to December 1901.