Comment by marcus_holmes

I only worked with the team making changes to the billing system (and even then, I only maintained a database of code modules, who worked on them, and what changes had been made - this was before git and we did version control painfully). As you can imagine, the billing system was definitely not going to survive the date suddenly being 99 years older than it was last month. So I don't really know why the rest of the system would fail.

But the project management team were extremely careful about only changing parts of the system that needed to be changed. Partly so that the scope was contained and second-order effects limited, and partly because the people making the changes were being paid vast sums to do this, and any reduction in work was saving real money. So when they say that it would all have stopped if the work wasn't done, I believe them ;)