My boss back in 2013 asked me to figure out why he was getting birthday notifications from his pet social media project almost a week early. It turns out the previous developers had written their own date library in which every month had exactly 30 days, leading to a year that was 5–6 days short of what it should have been. Apparently those morons didn’t know that some months have a different number of days than others. Or that leap years are a thing. Or that there’s a standard library full of tried-and-true functions that handle these kinds of things for you.