eg for the "Suffrage" badge ("Used 30 votes in a day"), when does the day start?
(Alternatively, it could be based on a rolling 24hrs, but I doubt it because that would seem unnecessarily complicated!)
I believe it starts at 0:00 GMT. To find out if that is correct for you, try voting 30 times and see what message you get. It should specify the time until you can vote again, +-1 hour. Check back again around then and see how many minutes until you can vote again and there's your answer.
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Stack Exchange runs on UTC time. You can see this by hovering over your name at the top of the page - the time is at the bottom of the profile popup. – Rebecca Chernoff Jan 16 '12 at 5:15
