Origin
On May 2nd, 2021, Redditor Jacob_potatoes posted an image macro in /r/dankmemes with the caption “Time traveler: moves a chair / The Timeline:” followed by a picture from Danger 5 in which Adolf Hitler says, “Adolf Hitler, FBI.” The post gained over 92,000 points (shown below).
Spread
Over the following month, multiple Redditors began using the template to make jokes. On May 4th, 2021 Redditor hin2u posted another variation referencing Hitler to /r/memes, gaining over 53,000 points (shown below, left). On May 24th, Redditor Baksteen_zas uploaded another Hitler variation to /r/dankmemes, gaining over 21,000 points (shown below, right).
Other variations took less political approaches, offering subtle changes to the modern world. For example, on May 30th, Redditor erenjaegerfan posted a variation showing black moving first in chess to /r/memes, gaining over 36,000 points (shown below, left). On May 26th, user FenixDriver posted to /r/dankmemes a variation showing every member of the cast of Scooby-Doo all dressed as Shaggy, gaining over 7,400 points (shown below, right).
Source: https://knowyourmeme.com/memes/time-traveler-the-timeline