[MayflowerMadam]

It doesn’t. Inauguration Day is the actual deadline, as it’s the only deadline in the Constitution. All other deadlines like Safe Harbor are superseded by the Constitution.

[FreedomPoster]

Even that day wasn’t adhered to in 1876.

[Liz]

WIKI——Safe harbor (law)

A safe harbor is a provision of a statute or a regulation that specifies that certain conduct will be deemed not to violate a given rule. It is usually found in connection with a more-vague, overall standard. By contrast, “unsafe harbors” describe conduct that will be deemed to violate the rule.

Theoretical justifications
Safe harbors have been promoted by legal writers as reducing the uncertainty created by simply employing a vague standard (such as “recklessness”).[1] On the other hand, this type of rule formulation also avoids the problem of creating a precise rule that leaves a judge with no available discretion to allow for “hard cases.”[2]:14–21

In theory, the safe harbor formulation can combine the virtues of vague standards and precise rules, allowing legislatures to prescribe with certainty the advance outcome for specific foreseeable cases, and to leave to judges to decide the cases that remain.[2]:16–18

[Boise3981]

All other deadlines like Safe Harbor are superseded by the Constitution.

The constitution gives Congress the ability to set the date/deadlines regarding federal elections. Congress did so with the Electoral Count Act of 1887 - which creates safe harbor day and the electoral college vote day.

Safe harbor doesn't appear in the constitution directly, but it exists in federal law as a direct result of powers expressly granted to Congress.

Congress could, theoretically, change the law. But that's not going to happen.