The answer is almost surely exactly 75 years from the exact moment of the author's death, by whatever designation that is given in different parts of the world, although it would be a rare case indeed where this was material.
The answer does not depend on where the work was published so long as that country is governed by U.S. or U.K. copyright law or the law of another country with the same statutory copyright duration at the time of the alleged infringement.
An author of a book dies the 1st of January of 2000, in London, at
2:05 AM, GMT. Yet, from the perspective of Washington, the author died
the 31st of December of 1999, at 22:05 PM, EST.
The copyright expires January 1, 2075 at 2:05 AM, GMT which is exactly the same as December 31, 2074 at 22:04 PM, EST, which is exactly the same time everywhere in the world.
An author of a book dies the 1st of January of 2000, in Washington, at
2:05 AM, EST. Yet, from the perspective of San Francisco, the author
died the 31st of December of 1999, at 23:05 PM, PDT.
The copyright expires on January 1, 2075 at 2:05 AM, EST and on December 31, 2074 at 23:05 PM, PDT, which just happen to be exactly the same moment in time everywhere in the world.
If an infringing act happened before this moment, then the moment at which the infringement happened would determine that date in the jurisdiction where a lawsuit would be brought that the statute of limitations would start running. Assuming for the sake of argument that the statute of limitations is 3 years in the jurisdiction in which you are suing in both 2074 and 2075, the lawsuit arising from the infringement would have to be brought by the last moment allowed for filing lawsuits on the day that is the third anniversary of the date upon which the infringement occurred in the jurisdiction where you decided to bring the lawsuit.
This said, the answer is immaterial first, because damages would be trivial for an infringement lasting only a few hours or minutes, and second, because it is highly unlikely that copyright will be the same in 2075 CE as it is today.