“Congress instituted ‘war time’ by law in 1942,” explains Nicholas Lewis, a historian with the National Security Research Center, the Lab's classified library. “It actually had a precedent from World War I, instituted in 1918. That law was repealed after that war, but that was where the time zones we know today originated.”
The 1942 law standardized the time change across the United States. “During the World War II period of daylight saving time, or ‘war time,’ nicknames of the time zones reflected that change, being called Eastern War Time, Pacific War Time, and so on,” says Lewis.
He adds, “The various Manhattan Project sites would have followed their local time zone’s ‘war time.’ So, Project Y at Los Alamos would have been on Mountain War Time.”
For a deeper dive into the history of time changes, read this article from the Department of Defense.