Unfortunately, there is no way to fix this problem through resetting. Restoring, DFU mode will not fix this. The only way to fix this is to run your iPhone's battery completely flat. Like, literally FLAT. You would need to let the battery run out, and leave it without charge for a couple of weeks. Otherwise, if you're brave enough, you can disconnect the battery of your iPhone, let it sit out for a couple days, reconnect it, and all should be good.
Computer devices run on values of zeros and ones, also known as a binary. So computers count time in a code of zeros and ones. Softwares contain this data. Setting your time to January 1, 1970 basically tells the iPhone that the time is a value of 0 flat. January 1, 1970 is when time counting moved on to 32-bit, and started counting seconds from 0. Setting your iPhone to that date back then is causing the iPhone to tell itself that the current time is somehow before January 1, 1970, which is a time counting system that is not registered on the iPhone's software. And thus, its causing your iPhone to be stuck in an infinite loop of rebooting. I assume that its doing that to try to figure out the time, which it can't because the iPhone's software doesn't have computer timing data for any time before January 1, 1970.
This is just what I'm assuming is going on. I'm waiting for Apple to release an official statement of the issue.
So anyways, if you drain the battery flat and reconnect it, I'm pretty sure the iPhone will start counting from time "0", which is January 1, 1970, which is a time that is registered in the iPhone's software.