time you join one table with another the linking keys need to be unique.

Timestamps aren't really unique. Sometimes databases right a bunch of records in the same instant. Sometimes data needs to be retroactively entered. It needs to have a timestamp, which is often midnight.

You can sometimes get away with linking keys that are "usually unique", but I'd only do it temporarily in a pinch. I've seen it blow up too many times. They are "I told you so" moments when I have to bite my tongue.