Can Bullet Time damage Baron Nashor if Baron is at a battlefield?

Yes — Bullet Time can damage Baron Nashor.

Bullet Time doesn't choose or target units; it targets a battlefield ("deal that much damage to all enemy units at a battlefield"), and the units there are selected programmatically rather than chosen (Rules 355.10.b / 355.10.d). Baron's "I can't be chosen by enemy spells and abilities" only stops effects that require the caster to select him as a target (Rules 756–758), so it doesn't apply here — unlike Alpha Strike, where you pick individual units.

So if you choose the battlefield Baron is sitting at, he takes the damage along with every other enemy unit there.