In leaflet.html, the hrefs are hardcoded to use the file protocol.
And in utils.py, the value of filename is generated as an absolute path.
This makes it impossible to publish the final result to a web site later.
I would like to suggest removing the file protocol and using a relative path for the picture location.

var popup = '

' + a.properties.date + '

' + '

';
marker.bindPopup(popup);
markers.addLayer(marker);
}
map.addLayer(markers);
</script>

It would be nice to intuitively distinguish each year/month by color.

Perhaps something like http://www.jacopocolo.com/hexclock/ ?