A native Floridian once told me that the New Year's fireworks display would be starting at “12 PM”. Confused about why they’d be shooting off fireworks in the middle of the day I said, “12 PM!?” She rolled her eyes and made air quotes with her fingers and said, “Well, AM”, as if I was being extremely pedantic. To her, AM meant morning and PM meant night.

Having worked on the UI of an event-related service, I can say the error rate is much lower if you go with the ‘11:59’ workaround. In addition to the confusion about whether 12 PM is 00:00 or 12:00, there’s confusion about which calendar day 12 AM falls on. When does a Friday midnight movie start? 99.9% of people will show up at the end of Friday, but in a literal user interface you’d have to choose 12 AM Saturday. I thought of fireworks girl very frequently when I worked on that UI.