This absolutely floored me! I just discovered this today when my iPod Touch and iPad went off for the same alarm more than a minute apart! I changed the iPad's time manually to match the IPT, but that's also wi-fi, so they're both probably off now! No wonder I've been outside waiting to watch the International Space Station go over and it hasn't appeared at the right time! Those astro apps need a synching time option (some may have it, in fact)--or, provide an alert to users.
Of course, what's really needed is for the iOS to fetch the time automatically on all I-devices!
By the way, ISS viewing is spectacular. It's done with the naked eye, and it is just amazing to watch this very bright "star" gracefully move across the sky. Well worth doing--the ISS Visibility app is highly recommended. Also, see heavens-above.com where you can also enter your location and get a list of upcoming passages and where in the sky the ISS will be.