The problem with that is you lose the localtime information, which you may want. Say you have recorded the localtime with the known UTC offset of 0, and that localtime is midnight UTC. Now you want to map it back to a localtime in the past. Ok, which localtime? Was it 8pm in NYC? Or was it 5pm in LA? Is the assumption that the localtime is wherever the user is right now, and is that a valid assumption? If the user has traveled to a different timezone, the time is now being converted to a localtime in the past that is different than it actually was when it happened.
edit: another commenter shared this link with an example: https://codeblog.jonskeet.uk/2019/03/27/storing-utc-is-not-a...
I like their "Principle of preserving supplied data".
