Hi all, long time lurker, first time poster.
Just thought I'd share my experience as it may help someone in a similar situation. I'm almost certain the OP didn't have a defective unit. While I can understand their frustration, it's something you can resolve!
Bought my ATV4 yesterday and set it up last night (eventually). Upon initial setup I received an 'unable to set date and time' error just after inputting my wifi details. Tried everything - resetting router, ATV reset, connecting via Ethernet. Same error every time!
Now I must say my abode is awash with Apple gear - never Ever had a problem setting anything up - MacBooks, iPads, iPhones, ATV3 - it just works right? (my router is even a current gen AirPort Extreme).
Was about to give up as a dud unit and return to store when I came across some forum posts regarding users having similar problems. I think some routers block certain ports - may be wrong - but port 123 is used to communicate with a network time server (hence unable to set date/time) and this was blocked.
Quick solution - use the wifi hotspot on my iPhone to connect the ATV to this network, finish the setup process and switch back to my home network again once finished. Worked a treat!
Hope this helps somebody out there with similar problems.
Very very pleased with my ATV4 by the way!