I am looking for a veryshort story from the 1960s about a time traveller who travels back in time fifteen minutes only to tell the person he meets that the amount of energy it took to propel him back in time destroys the universe.
This is "The Man from When" by Dannie Plachta.
It first appeared in Worlds of If Science Fiction, July 1966.
You correctly recall that the story was very short (only 2 pages, pp. 77-78). However, you incorrectly recall how far back he travelled. It was 18 minutes.