I work with industrial control systems and the oldest code comment that was found about the year 2000 problem was in code from the early 80s. The programmer long retired but that code was still running oil refineries.
There were very little large chemical installations in the pacific so we waited for updates from sites in New Zealand. After those sites rolled into the new year without problem everybody relaxed and we knew that our software installed in middle east oil&gas sites would work ok so people would have petrol and diesel in the new millennium. Then the rollover in the EU and US region were easy after that.