So modern technology is literally erasing our pasts. Not just calendar entries, but messaging systems (people used to keep handwritten letters for decades), and possibly even photos (if we're not careful about preserving them).
Edit: See my clarification of the 2 years in comment https://news.ycombinator.com/item?id=30084620 below. I still think the point remains - we do not own or value our digital data in the same way as physical objects, and there is a much heightened risk of that data disappearing as a result, either by the owners of the platforms the data is stored on archiving the data or by us not valuing it enough to preserve exports and backups through long periods of time.