It is a fairly common practice for books to hold the games open a few extra minutes. Just prior to kickoff, many phone systems are jammed and players get a busy signal or placed in the queue. Rather than deal with the $50 players complaining that they were going to bet $500 (on the winner) had they gotten through, it is much easier to keep the games open a few minutes.
There are also books that have computers that are set a few minutes slow or fast. So, InSpades, it may well be that the book recorded the time of the bet as before the official kickoff (if their clock was slow).
As long as there was no score prior to the bet, they should accept it.
Now, if you lose the bet, you should send them an email asking if your bet is still good and get their OFFICIAL policy. To save you the suspense... "blah blah blah If we take a bet, it is final. blah blah blah"
Of course, you get that letter in VERY strong terms when they want to keep your losing bet. So, if you lose, pretend to fight it a little bit, just enough to force them to state their policy.