Hi There Mahlon,
Not sure If I was just misunderstanding the Dialog here - But after going back and testing myself, It looks like we've already added the 'Compare As' For DateTime fields on Chart objects.
That being said - I think your initial question was more along the lines of "How do I display DateTime as JUST Date, without the TIME element" - This woulds till take a workaround to implement properly.
Thanks Mahlon.
Regards,
Ben