Randall Garrett

“ Evidently, he'd been doing something the past two weeks—but what?
He remembered talking to Kamiroff about the variability of time—something about a man named Dunne. And he remembered the biochemist saying that time travel was physically impossible. For a second or two, Bethelman wondered whether he'd been projected into the future somehow. But if he had, he reasoned, he'd still be wearing the same clothes he'd had on in Boston.
No, he decided, it's something else. I've gone off my rocker. I'm daffy as a dung beetle. What I need is a good psychiatrist. ”

“ Now you'll relive those two weeks."
"That sounds like a weird explanation," Bethelman said.
"Well, look at it this way. Let's just say you remember those two weeks in the wrong order. The drug mixed your memory up. You remember the fortnight of the second to the fifteenth after you remember the fortnight of the fifteenth to the twenty-ninth. See?"
"Good gosh, yes! Now I see how I made all that money! I read all the papers; I know what the stocks are going to do; I know what horses are going to win! Wow!"
"That's right," Kamiroff agreed. ”

“ Elijah Kamiroff floated above him.
Bethelman sat straight up in bed, his eyes wide. The effort made his head hurt worse. He looked around.
He was in the upstairs guest bedroom of Dr. Kamiroff's suburban home.
He turned to look at the biochemist, who was busily mixing a bromo.
"What date is this?" he asked.
Kamiroff looked at him with mild blue eyes. "It's the second," he said. "Why?"
Bethelman took the glass of fizzing liquid and downed it. The pattern was beginning to make sense. He had gone to sleep in Boston the night of the first and awakened in New York on the fifteenth. ”