During world war 2, much of the the US Pacific fleet was operating in different time zones. If say an attack was to take place at 7am would it be 7am on the island? Or 7am where the Navy is? Or 7am at HQ?
If that makes any sense.
World War II Pacific Island Guide: A Geo-military Study by Gordon L. Rottman, specifically for WW2, states that for US forces, local operations and reports typically used local time while Japanese forces used Tokyo Time. But for bigger operations, US forces would use Zulu time (Greenwich Mean Time) since forces would be spread over larger geographic areas and potentially multiple time zones.
This practice is still in use today, of course. You'll often hear military talk about Zulu time. And times are often written with an identifier of a time zone in any message that crosses multiple geographic areas. For instance, modern US Navy message traffic still puts times out like 0031Z with Z stating that its Zulu time.