Additionally, since the timestamps are accurate to margin of a few hours, if you establish a proof of existence with a transaction in a block, you can confidently state the year, month, and day so long as the block doesn't occur within a few hours of the border between 2 days, months, or years. If you add a few hours to the timestamp, you can confidently say that the proof was established no later than that.
This.
You can say, with confidence, that something existed as of a certain date. Or before a certain date. Or just state that the accuracy of this time stamp is +/- 2 hours. When people have something notarized by a lawyer or some legal office, magistrate, judge, or public official, they are accurate only to the day. And even then, there are some offices that pre-date or back-date stuff.