You can still do it. I called a few weeks back when I was dead set on moving to (shudder) Comcast (overall package saved about 600/yr). The nice folks on the loyalty line 1) made it worthwhile to stay and 2) let me set my cancel date almost 2 weeks into the future. Those 2 weeks gave me time to get my head on straight and take offer 1.
Yes, you can set a date. When I moved to Time Warner recently due to their great win-back offer the Dish rep asked me what date I wanted the programming to end.