and if you're still not thinking this is fun, here's a quote from Wikipedia "But keep in mind that "PT36H" is not the same as "P1DT12H" when switching from or to Daylight saving time."
Maybe surprising for days (could also happen with minutes because of leap seconds technically :-) but for months and years this is more apparent due to month ends and leap years.
Oh wow, never looked at ISO8601 durations before and I had no idea they were this ugly. Please, no, don't make me deal with ISO8601. I'd rather write a number of seconds or a format like 'X weeks' or 'Y hours Z minutes'x ISO8601 looks exclusively like a data interchange format
I learned it too. It's just ugly. Before I looked at the relevant section of the ISO 8601 duration format yesterday, I didn't know it. After I looked at it, I now know it, and I strongly dislike it.