Oh, that snippet has been there all along. In fact the (registerDate) value is displayed on the front-end if I manually add it via the Manager.
So the problem isn't the placeholder or the ClassExtender field, it's that my PHP wasn't generating the date, but I found the problem. I changed it from this:
echo date('F j, Y')
[ed. note: todd.b last edited this post 3 years, 4 months ago.]
echo date("F j, Y");