I just use 24 hour time, so am/pm isn't necessary. I do recall once, though, long ago (about 35 years ago) taking a nap, and waking up at about 6:00 and having no idea whether it was morning or evening. I had no clocks that indicated am or pm, and I didn't know if I needed to get ready for work or for something else. I finally turned on the TV to find out the time of day. At certain times of the year, at 6 it's almost impossible to tell the difference quickly.