For that you would like to have an explicit "local time zone" option. I had recently similar in application I am developing, with datetime of which we are not sure in which time zone they are. That information might arrive later.
On my Palm Pilot there was a “floating” time zone that behaved like this. But I can’t find this feature in any modern app. Outlook doesn’t have it either.
Apple's calendar has it on macOS, but not on iOS... (it's annoying how mobile everything always has to be so neutered)
And yet iOS calendar is also more powerful in that you can set different time zones on the start and end of a single event, which macOS calendar will show but not let you edit.