Is the Venice vaporetto pass based on time of purchase or is it per "day" for the 24 and 72 hour passes? And, yes, I am an idiot and accidentally posted the same question in the "General Europe" section. Sorry.
I was in Venice recently and I may be mistaken, but I believe a 1 day pass is for 24 hours from the time you buy it. eg. If you buy one at 1pm Thursday, its valid until 1pm Friday. They are sold as one day, three days etc. What makes me think this is that when I went to board the vaporetto at the train station there was an inspector who looked very closely at the stamp on my ticket. Must admit I didn't bother looking as my pass was for longer than the time I was there. I'm sure someone else will know for sure.
I bought my vaporetto pass from the info booth at the train stattion because if your under 29 you can buy a rolling Venice card which saves a lot of money, so I got a 3 day pass for only 15 euro instead of 25! Once you are given your ticket you go and stamp it in one of the machines before you board the vaporetto and it stamps a date a time so if you stamp a one day pass at 9am on a tuesday you can use it till 9am wednesday, they do check. I found the three day pass the best deal because my hostel was near the train station and I just loved takening the vaporetto, its fun to hop on and off as you please.
Yes--the passes are for a specified number of hours, so the 24 hour pass is valid from the time you stamp it one day to the same time the next day.