I've never heard of such a law. The Constitution clearly says when the presidential term ends and when the next begins. But it leaves the specifics of the presidential election to Congress. Article II, Section 1, Fourth Clause reads:
The Congress may determine the Time of chusing the Electors, and the Day on which they shall give their Votes; which Day shall be the same throughout the United States.
In practice, Congress has almost always deferred to the states when it comes to election law. But in theory, Congress could pass a law that changes the date when the electors are chosen (i.e., the general election day, this year on Nov. 2) or the day the electors meet, as long as all states did it on the same day. All of the state legislatures would have to exercise their own prerogative and choose their electors too, which, as mentioned above, they would really really try to avoid doing. Even then, most states don't require their electors to vote for the candidate their committed to. So again, in theory, if there were a serious national emergency after the electors are chosen but before inauguration day, the electors could elect whomever they wanted, as long as he/she met the constitutional requirements to be president.
Pretty hypothetical though! :blink: