Congress can delay an election in a national crisis. Or they can delegate that to the Executive branch. However, hard dates have to be given.
Check this article out for more information.
https://constitutioncenter.org/blog/does-the-constitution-allow-for-a-delayed-presidential-election
I am aware that Congress can do it. Article II-Section 1 - Clause 4. I should have been more clear: The idea that HE can do it is unconstitutional. Now - I know that is not what he said - and right now the date is set and it would have to be changed by the congress - which we know is not going to happen.