As an intern I was once given a spreadsheet of manually entered daily data, stored by a human being over the previous couple of years. Obviously he'd entered the dates in whatever form he felt like at the time and Excel had parsed them in a variety of godawful ways not least of which was defaulting to American month/day assumptions before UK day/month.

I think it would have been far quicker to just manually write a new column interpreting the dates based on previous/next etc. Instead I spent God knows how long trying to be clever, failing, and being embarrassed that I could not solve this obviously trivial problem.