Gregory's ServerMy experience was that I spent a very dull 6 months going though COBOL code, checking for date fields. Changing YY to YYYY.
They may have been non-critical systems but a lot of the telco's billing could have fouled up and caused financial issues.
Unix code has its own Y2K38 problem. Could be lots of last minute work for programmers on those legacy systems.
https://en.wikipedia.org/wiki/Year_2000_problem#Similar_date_bugs