JW Tumblr Stack - Date format

I would like to change the date format shown above the blog panels to the more UK-friendly dddd, dd mmmm, yyyy. Does anyone know if this is driven from the Tumblr account itself (where I can find no way of changing it) or, if not, whether it can be customised somehow within RW using CSS? Having checked the Tumblr stack CSS sheet it does not seem to contain a formatting element.

Thanks all