Justin's basically in this part of the storyline to sit there and look pretty.
Elliot's actually wrong regardless of when precisely Susan was born on New Years Day because the ball he speaks of is the Times Square Ball
, which is located in New York City. Moperville is somewhere vaguely Midwestern and has a Central time zone, so "when the ball drops would actually be 70 minutes early, not ten.
Well, that, or he was just trying to say "midnight" in a fancy way and wasn't being literal in referencing the iconic ball, but it's fun to nitpick.
Incidentally, if there is a scene with them singing to Susan, it won't be the famous "Happy Birthday" lyrics. That song is sadly not public domain (which is always shocking when first learned, but the knowledge has become more and more common). On the plus side, it will be an excuse for me to come up with my own horrible lyrics.