There’s already a feature request for that: https://github.com/pixelfed/pixelfed/issues/2871
So far, I only know of GotoSocial that allows backdating new posts. And it even has a “gallery” theme that you can set as default to get something remotely similar to Pixelfed.



Manually backdating is one thing, but importing them from Instagram with the original timestamp is a different. That’s why #2871 is still open and #4558 was closed as “not planned”, I guess.