I think I know what it is, it seems to be related to the language/locale, perhaps it's not matching your system locale, so GTK+ can't figure out how to format the date and just puts Unknown instead. Can you paste the first few lines of `Help->Debug Messages`?
I can reproduce if I set the `LANG` environment variable to `C`, but not when I use a proper locale like `en_CA.utf8`, `en.utf8` or `de.utf8` or the default. My Windows is set to `English (United States)` language/locale.