This brings back a bad memory!
I worked in the London branch of a US investment bank, in the IT department, and the Ops department had hired one of the Y2K COBOL "consultants" we had been using prior to Y2K to "knock them up a system to help record certain compliance issues". So this all took place just before Y2K (Oct/Nov 1999) as the consultants were all looking for their next gig by then and had obviously convinced one of their old users (who had been system testing Y2K) that they could help them out with some of their more mundane reporting issues.
Apparently, development of the system had gone swimmingly and the consultant delivered the system (with no input from IT, this was all done behind our backs) and the Ops department - which had a mixture of UK and US staff - diligently entered their data for a few months, and the consultant left the scene rolling in dough.
Shortly after, early Jan 2000, the system began misbehaving - the reports weren't making any sense. Could the IT department send someone down to fix it?
We had a brief internal discussion within IT, mostly involving the phrases "fuck off, I'm not touching it", "no fucking way am I taking a look" and "they developed it, it's their problem!" until finally I got given the short straw and talked into taking a look, but that if it couldn't be fixed relatively quickly then I should leave it alone as it really wasn't our problem (apparently the Ops department had been warned about this sort of shenanigans in the past - mostly using massive Excel spreadsheets that nobody could support).
So I went down to Ops. Asked to see this system. All based on Access 97 and Forms. I quickly ascertained what it did, and what the problem was. In a nutshell: "A lot of the dates are wrong".
I began looking at the data, and deduced that the Access 97 database being used to store the application data was using a TEXT column for all of the dates.
You can all guess where this is going... right?
Some of the users entering the records had their PCs set to use UK dates (dd/mm/yyyy) and other - US users based in our UK office - had their PCs configured to use US dates (mm/dd/yyyy). Sigh.
"01/04/1999" as stored in the database could have been 01-April-1999 or 04-Jan-1999. Was "01/04/1999" entered by a UK user the same as "01/04/1999" entered by one of our US visitors? Yes? No? Maybe? Who could possibly tell anymore?
And, due to the total absence of any audit trail, it wasn't even possible to know which user had entered which records (and thus dates) so matching US users to specific mm/dd/yyyy records - which might have been the only possible way to unfuck about 50% of the dates back to a sensible dd/mm/yyyy date format - was entirely impossible.
I swiftly threw up my hands and said the existing data was now garbage, and couldn't be saved. In future, make sure everyone has the same date format on their PC when using this system. Bye!
As for how it ended, the US investment bank was bought out by a European bank about a month later, and the Ops team (and their noddy system) were made redundant. That, as it turned out, was a lucky escape!