It’s another one of those ‘strange’ problems that has cropped up, but by default in Plone, users who aren’t authenticated aren’t actually able to bring up a pop-up calendar on a date field.
It’s amazing that I haven’t run into this issue before with something like PloneFormGen, where you’d think that a simple Date input field would be pretty common. Although, I guess it’s the case that even though I’ve used these given forms, I’ve probably never taken much notice of the popup calendar icon and just selected a random date from the drop-down lists. I suppose entering a random number on they keyboard and hitting tab across the fields 3 times is simpler than going for the mouse. Who knows.
The solution was the only Google result at the time, and that was at: http://plone.org/products/ploneformgen/issues/232