If a bill that was passed by the U.S. Congress is signed by the President, and the bill named an effective date, at what instant does the law take effect? For example, if a provision is set to take effect on January 1, 2021, what time zone determines when it is January 1? Is it Eastern Standard Time, because Washington, D.C., is in that time zone, is it dependent on the local time in each part of the country, or is it determined by some other time zone, such as UTC?
Many laws contain a clause which describes how they take effect. There might be some trigger or they might come into effect at a particular time, as described in the bill. It is possible that they contain a trigger that specifies "11:59 on December 31st" In which case they come into effect at that time in each timezone. This means that they are in effect in Maine before they are in effect in Hawaii.
If the bill doesn't contain such a trigger, they come into effect from the moment that they are signed by the President (relativistic effects notwithstanding).
This could mean that there is a period in which something is legal in Hawaii, but illegal in Maine. If the bill created a criminal offence, then part of the job of the prosecution would be to prove that the act took place at a time and place in which it was illegal.
For bills that, for example, permit spending, the relevant time is the time in Washington DC, since that is where the President's office is, and so that is where the money will be spent.
This is not normally a problem, and if it is forseen as one, then the bill can easily contain a time zone it its trigger clause.