This tool lets you migrate your own posts from any subreddit to any Lemmy community you want, I made it because I couldn’t find anything else that did it the way I wanted to. I’m super happy how it turned out, I managed to get 9 years worth of posts over this way…
The GitHub link shows how to use it in case you want to!
It will keep the current date due to users not being able to override a timestamp, but the top part of a post shows the original date and link to the original post: https://lemmy.emerald.show/post/764