Most of car rentals have a contract with other companies where companies get various discounts and addons in exchange for continuous business. It is good deal for everybody involved. But sometime curious things happen.
For example company I work for has benefit of “Monthly charge up to 31 days”. Simplified it means that your rental comes at cheaper price a bit earlier. For me that has an even better benefit of getting reimbursed on month basis. In ideal world, if I rent car on 1st, I get an receipt on the same date next month. Except it isn’t so.
For some reasons National has also an policy of closing each contract every 30 days. If you imagine a world where some months have a day longer you will see a problem. Let’s assume my rental starts on September 1st. First bill is due on October 1th. Next bill after that is October 31st. One after that is November 30th. Congratulations, you have just lost a day.
In reality this is not really an cost issue. Although you have monthly rate actual calculations are still done per day so everything evens out. However, person has an interesting choice between sending to accounting two bills for all months that have 31 day or sending them “monthly” bills that end on funny dates.
It only shows that for system design you must consider edge cases (and test for them) or users might get confused…