date format DD MMM YYYY

I believe you will have to use a calculated field where you can set the format. There is a post about doing so Formatting the Current Date in Nintex Forms

The Format of the date displayed in the date control is dependent on the regional settings of the site collection.  The string format field in the configuration only adjusts the format of the date in view mode.   Entering the date will be in the default format but putting DD MMM YYYY into the String format field in the configuration of the date control will display the date as you require but only in view mode of the form.