Hi ,
We also dealt with HiFX but our time limits were only days- ie we got a rate, and the money should have been there within 48 hours, we explained that it would take our bank 3 days to get the money there and they accepted it.
Therefore if you're only going to be a couple of days out, it might not matter, if you're more than that they may give you a lower exchange rate, dependent on the rate at the time ie if the rate is better, they'll stick with what they offered you originally, if worse, they may cut your rate.
I think that you should give them a quick call and explain your position, I've found them very helpful, even on general enquiries.
Good luck!!
Sharon