What time is the exact time to start the test? do we need to start at the exact time or can we have a time window'? Ok Is the test meant to start at 9.00 sharp? or can we delay it some minutes?

What time exactly must we start the test? Do we need to start at the exact time or can we have a time window? ("time window" sounds a little odd to me but it might be okay, I would say "leeway" or "flexibility") Is the test due at 9.00 sharp? or can we delay it a few minutes? (These look like pretty good sentences, it just needs a few small changes.)