"how do I make the datefield conditionally required"
Do you want to validate it on client or server?
If you show/hide the fields on client side as and when required and submit a form, the hidden fields will NOT be submitted to the server (AFAIR), so on the server you can just check when the fields are present in form scope...
I was hoping to be able to use the 'required' attribute like I am already using on a few other fields.
Basically, If the 'yes' radio button is selected then I want the datefield required.
Yes = Required datefied
No= Not required datefield
You can write some js for that, but it's not likely to solve the problem in the OP.