There is really no set date. There is a mining difficulty bomb that is set to make ETH mining unprofitable by increasing the difficulty. It is to be triggered (If I remember correctly) Q2 2019.
As for now I have seen this from the GitHub: https://github.com/ethereum/wiki/wiki/Proof-of-Stake-FAQ
and this other article: https://www.coindesk.com/ethereums-big-switch-the-new-roadmap-to-proof-of-stake
R&D must be pretty difficult. These guys are changing the wheels of a moving car.
Thanks!