Alright. I think I cracked it.
<dateTime.iso8601>20140111T20:39:15</dateTime.iso8601>
Should be:
<dateTime.iso8601>20140111T20:39:15Z</dateTime.iso8601>
Looks like the toolkit I was using didn’t quite implement iso8601 perfectly (or wordpress isn’t – not sure). I’m using Node WordPress and tracked down that it uses the module Node XML RPC to actually send the XML RPC. This seems to be where the wrong date formatting is occurring.