Once you set your iPhone’s date to Jan 1, 1970; it will stop working. Even restoring your iPhone using iTunes won’t fix this issue. So, I recommend you not to set date to jan 1, 1970.
Someone might take your iPhone and can set back to 1970. So, be aware while giving your iPhone to other people.
1. Let your iPhone’s battery run out so it does a cold restart. This will reset date.
2. Open your iPhone and disconnect battery. Once done, connect again. This will reset date too.
Warning : This will voilate iPhone warrently.
3. Go to Apple store and ask them to fix it.
Thanks you for reading. Hope this might help you. 👍