Impossible to format dates in list-view filters?

Hi folks,

I'm stuck trying to do something fairly simple. I hope someone with more Contao development knowledge can tell me if what I want to do is possible?

I've switched on "Deactivate on" as a filter in the member DCA:
$GLOBALS['TL_DCA']['tl_member']['fields']['stop'] = array
(
'filter'=> true ... )

... but although the filter appears and works, the values shown are UNIX timestamps not formatted dates (see attached pic). I'd like to ideally pass some sort of formatting function through and have these shown as formatted dates instead.

Is this possible via the DCA, callbacks, etc?

Thanks!
Howard.