Hi there. Just add the following code to your HasOneComplexTableField.
$tablefield = new HasOneComplexTableField();
$tablefield->fieldCasting = array(
'Date' => array("Date->Format","d-m-Y"),
);
You can use array("Date->FormatI18N","%d. %B %Y") for a even nicer date too.
http://api.silverstripe.org/2.4/forms/fields-relational/ComplexTableField.html
http://api.silverstripe.org/2.4/sapphire/model/Date.html#methodFormat
http://api.silverstripe.org/2.4/sapphire/model/Date.html#methodFormatI18N