Hmm, I don't think so, if my memory serves me correctly, but don't quote me on that. I believe I've seen them during the day. But, they are rare to see them trotting around.
I noticed that the Skeletons in the ruins of Centaur Hills popped after the server message: Night Falls. (I believe that's 8pm game time)
This is one of the many little quirks that's making me really like this server. Why would the undead rise from their graves in the middle of the day? Of course they prefer to haunt at night....