Using data within a mixed attribute type to query Craft entries dates
Just in case anyone else comes across this problem in the future I actually managed to figure it out myself.

I created kept the fieldtype as a DateTime field and grabbed the date/time and the extra value within the prepValueFromPost function then pushed the extra data into a custom table and post just the datetime to the craft_contents table.

Then within the getInputHtml function I grabbed the extra data from the custom table to display along with the date/time.