That's true if you catch the exact moment the clock changes. If you don't, the only thing you know with a truncating clock is that the fireworks started 0 to 59 seconds ago. With a rounding clock, you know the starting point is within [-30, 30] from now. So on average, you're closer to the starting point when seeing the clock show 12AM.

A good reason for truncating is that we have a strong bias against being late, but not really against being early.