[Xycl]

... I think it would be awesome if there was another category called "Unknown date"?

Possible with latest release.

Also, do you think it would be possible to browse photos by year, then month and then day?

Already possible

And to see thumbnails instead of a list of photos? It may depend on the skin, but I am basically throwing ideas.

You can change the view type. Possible types depend on used skin.