That's not a realistic comparison. More realistic would be if Gandalf said "If the ring is not destroyed by Friday, Sauron will definitely defeat us, and any effort spent on trying to get to Mount Doom will be wasted, and may cost us the chance to defeat Sauron a different way. Given that, Frodo, can you destroy it by Friday?"

Of course, in most shops Gandalf will keep all of that to himself because he hopes that Frodo can get it done sooner (it's just walking, after all) and he doesn't want Frodo to half-ass it, take his time, and not deliver until the last minute. So he'll just ask Frodo when it can be done.