Since Destiny is a global game the timing of the server resetting daily challenges can catch many of us unawares in other parts of the world. I've completed the daily story only to find I won't get my reward because I finished "too late". Since the daily is already restricted to one completion, I think it would be more fair to reward players based on the time they started the challenge, as opposed to finishing it. They still will only get the single completion for each daily in any case.