I was told the same thing as Steve a few years back by the rangers at the office while picking up our permits. As long as we didn't pass the sign before midnight, we were fine.
We started at 10pm and got to the sign just after midnight.
As for Bob's comment, we had to start that early because we had to drive back to Vegas directly after our hike and preferred to leave a few hours of cushion before our plane left in case anything went wrong. Leaving at 10 was great. We didn't see another person on the trail until just before the switchbacks.
