Yes. 23:59:59 is used when somebody wants to get end of day but doesn't know to take beginning of current day and add 1 day to it. End of today is actually 00:0

Yes. 23:59:59 is used when somebody wants to get end of day but doesn't know to take beginning of current day and add 1 day to it. End of today is actually 00:00:00 of tomorrow. For some reason developers don't want to have tomorrow's date in the timestamp when they think about events happening today but this is ok, because it should be used as upper limit (ie. with some kind of less than operator).