That being said, it is kinda useful for data conversion. Like, if I wanted to pull in old posts from somewhere else and stitch it together on some kind of ActivityPub blog, the dates could match the original creation dates.
In fact, I’ve been doing this with videos hosted on VidCommons. I threw an old John Wayne movie on there that’s like 100 years old, and the timestamp reflects the actual day the movie came out. Other ActivityPub servers reflect this.