My working theory on why this happens is that there are two different models and people sometimes have problem choosing the correct one. When you say "Thursday

My working theory on why this happens is that there are two different models and people sometimes have problem choosing the correct one. When you say "Thursday" you mean entire 24h period. It is not a point in time, it is a label for a span of time. But when you say "1 pm" you don't mean an entire hour, you mean a point in time that is more or less precisely 1pm. It seems people extend the first model to a lot of cases where it is not correct. And so for many people their mental model of 23:59:59 is a label for a span of time that is one second in length. If your model is that a day consists of 86400 "seconds", each second being a span of time of length of one second with a label like 12:37:28, then 23:59:59 is the last second of the day and 00:00:00 is the first second of the next day.