This was a bit misleading. It's about date handling, which is a complex scenario well outside 1 + 1.
I too have had my share of problems with date handling. One year is not always 52 weeks, A month is never 30 days, most days have 24 hours (but not all of them) and so on...
Not to mention the famous D/M/Y, M/D/Y, Y-M-D ordering problem...
Which meant that we where unable to collect any Direct debits for 3/4 of year!