I had one of these on my commute in Atlanta for a while. (Left turn) It was impossible to read the sign unless you came to a stop, at which point a cacophony of honking emerged from behind, whether it was the right time or not.
At some point they added a little yellow light to indicate it was legal to turn. I was a little awestruck.
I got a speeding in a school zone ticket and fought it because the points for that offence are high. The sign gave times the speed limit was in effect but I missed it and there were no other cars on the road so I didn't have that as a reference either. My lawyer argued that if the time of day was required knowledge there should have been a clock installed under the sign or some other visual alert. I got off.
At some point they added a little yellow light to indicate it was legal to turn. I was a little awestruck.