In theory, 5pm on my last work day (whether that is in the office or outside of it) is when my vacation starts, and 9am on my first day back is when it ends.

In reality, 4:30 tends to be the time that everyone remembers I am going on vacation and 'just needs' x y and z (despite them having been told a week earlier, two days earlier, and that morning). So I cannot actually recall the last time I left the office at 5pm the day before a vacation! Thus I work towards 5pm, and build a 'known only to me' soft cushion of a couple of hours just in case (ie I don't book at 7pm flight!) If I am out of the office, I try to arrange to be finished by 5, or plan to be back in the office that day - or, as I have done occasionally, with permission of the company, go straight from where I am (e.g. in London for business, fly direct to Europe for a few vacation days) If that is the case, I will be as flexible as I can as I know those things work both ways.