[fleetingmoments]

Couldn't the rollover be simulated ahead of time by simply setting the date forward? Seems crazy that you had to have people on the ground fixing things as the clock struck 0.

[salgernon]

All systems in the gigantic clockwork of interconnected legacy would need to be rolled simultaneously- and it wasn’t unreasonable to expect that there are systems in that mix that nobody knew could be affected. To this day I read about systems that come up as a surprise when they start to fail and nobody owns them and the persons responsible have long since lived on.

[Diederich]

Two decades ago, I was roughly familiar with the details, after the fact, but I can't remember now.

As sibling comments have noted: these systems are an exceedingly complex pile of separate but interdependent pieces.

The rollover from 31-Dec-1999 to 1-Jan-2000 had happened thousands of times before the actual day came. Some piece/assumption/etc had been overlooked.

[davesmylie]

Sure. On systems under your control.

It's the network of interacting systems that prevents this from being so simple.

[protomyth]

Yep, the super fun of getting a Y2K compliant specification of how the data will come over the interface only for it to fail and have '1910' instead of '2000' in the 4 character date field that worked in test. People on site to correct problems were a needed item for some even after testing.

[marcus_holmes]

You can do that with test systems, but not live ones. People object to having their bills dated 98 years ago and overdue on the day they were issued ;)