# Dates Are Hard Too

I mentioned a while back that one of the things developers make assumptions about is that time is a monotonically increasing function. If only that were so. But it's not just time, one second ticking into the next, that isn't as simple as it seems. Duration is hard too. Not just the number of seconds between two times, but how you turn seconds into weeks, months, and years. For example, the windows calculator can tell you how many days, weeks, and months there are between two dates. Seems like a relatively straightforward thing to do. But sometimes it goes wrong.

For instance, It claimed that between July 31st 2019 and Dec 31st 2019 there were 5 months, 613566756 weeks, 3 days (152 days). The 152 days part is right, but the weeks? How in the world did they come up with that number? Turns out counting is hard. I'll leave the details to the article, but it comes from mixing signed and unsigned numbers and ambiguities in what it means to advance a calendar by one month since months aren't all the same length. It usually works, but there are always edge cases. Something to think about as we work on safety critical systems.