The one for camp mccarran, right? It can be buggy, yes, but they may have fixed it....or not.
Anyway, if it's the one I'm thinking of, the quest should give you a hint what time...I think it's around midnight or one in the morning or something, don't believe it has to be exact, just in that general time frame. Make sure to wait at the foot of the stairs to hear the whole spiel before going all the way up.