In 0.10.0.0 of Tower Unite, if you achieve an achievement after the release of 0.10.0.0, and view it in the collection book, it states that the achievement was achieved 1/1/1970, at 12:00 AM.
This happened for every achievement I made after the 0.10.0.0 update. I was achievement hunting in the arcade ONLY when I spotted this bug. It is unknown if it happens anywhere else, but if it does, I will update this topic!
EDIT: I tried several other achievements in the Plaza and some games, and they also do in fact appear as unlocked on 1/1/1970.