Without having all of the network synced to the same clock doesn't this have the potential to fork the block chain?
If my 14 days is just slightly different then your 14 days and a block comes out in that gap of time it may be valid to me but not to you.
I'm not knowledgeable on the technicalities but I think timestamps take care of that problem. Even if you are out of sync when a block with new difficulty arrives you would see something like "time : 1412542299" and go "oh ok, that block was created at that time so it is valid". Could be wrong though, we'll have to wait for confirmation from someone who knows for certain how it works.