Hello John,
If you were to use Bill Gottesman's suggestion (a string tied to the gnomon's foot to artificially extend the shadow length), you could also eliminate all of those concentric circles marking date boundaries. Just put on the string a sliding bead which may be set to the desired date-distance, according to a scale of dates drawn at any convenient location on the dial. Of course, the scale would begin at the gnomon foot.
Best, Mac
