The ones in my city are all "School - Speed Limit 25 When Flashing", with a radar speed check display.
They must have an internal clock but apparently they don't have the school year in any way programmed in. They still flash at the appropriate times all through July and August and on holidays. Not on weekends, though, at least.
Also, some of these things are on streets where the speed limit is already 25 regardless of the hour or location.
Ah well, they still remind you how fast you're going, at least.
The best part, though, is the fact that these are all fairly new, and the old signs with conflicting info are still up in some places. So, you'll see "School - Speed Limit 25 When Flashing" followed right up by "School - Speed Limit 30" (note to self: get a picture of this)