One of the most irritating repeating problems in programming is dealing with humanity's absolutely insane calendar. 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day. Except for leap seconds. Somewhere between 28 and 31 days in a month, depending on the month, and the year. 365 or 366 days in a year - 365, except every fourth year, except every hundreth year, except every four hundreth year. I imagine there are more "excepts" that I don't know about.
Remember that these days are not necessarily all the same length.
AM comes before PM, except for 12:xx, which is reversed. Of course, it's not the same time all over the world - every place has its own time zone, most (but not all) of which are offset by exact hours. And some places even change time zones twice a year - for example, most of America, except Arizona and the east half of Indiana, which don't. (Except, apparently, the Navajo Indian Reservation in Arizona. Which does.)
Basically it's a mess, and while I can gratefully ignore about 80% of the mess, the 20% I'm having to deal with is seriously irritating me.
Which makes it even more annoying when I open my inbox and find an email titled "Everything you need to know about dating!"
PLEASE. TELL ME. SAVE ME FROM THIS HELL.