I've seen this issue posted in relation to ROMs, but I'm stock. In fact, this issue only occurred after a factory reset on my third Dx in 2 months (original killed by Gingerbread release, replacement had a hardware issue with the touchscreen, I set up the wrong Google account on the new one and had to reset and start over).
Now my calendar notifications show up AFTER the event starts, though they are set for, variously, 5 to 30 minutes in advance. They seem to show up about 18 minutes afterward (though I had one yesterday show up almost 30 minutes into the event). I've checked time zone (though that wouldn't account for 18 minute offset) and nighttime settings.
Anyone have any thoughts on what's going on?