Normally, it's the application date -- the one inside your form. So, even if they were to get the envelope on June 20, 2015, and your date is June 24, 2014, it will be processed according to old rules. This is what a CIC agent told me as well.
By the way, the training involved in the new provisions is extensive and expensive. That's why they'll be doing that over the next years. Also, more training will ensue with applications that will be received as of June 2015 (or even earlier), which might slow things down a little bit.