Lieberman and a Date Certain

So, apparently Joe Lieberman truly does have no shame, as he's using today's terrorist scare against Ned Lamont.

"If we just pick up like Ned Lamont wants us to do, get out by a date certain, it will be taken as a tremendous victory by the same people who wanted to blow up these planes in this plot hatched in England," Mr. Lieberman said at a campaign event at lunchtime in Waterbury, Conn. "It will strengthen them and they will strike again."

Here's my question. Is, as was suggested by Lamont's campaign manager Tom Swan, Lieberman just taking his cues directly from Karl Rove, or is there an intermediary?

And what the hell does "a date certain" mean? I heard Hilary Clinton use this phrase and it just sounds like a grammar abortion. What is wrong by saying "a specific date?"  Where did this come from?