I don't believe this is possible with Marketo... what goes into those fields is what comes out and you can't manipulate it like you might with PHP.
You could consider splitting them into two fields: one for the date and one for the time, then if you ever need to have both, you can just concatenate them, or you can use them individually. That being said, you'd have to split the values apart on your DB side, which I don't know if you could do.