Convert date entry to UNIX timestamp

rkv_date_convert

// Make two admin-only fields (as text fields)
// and post to the custom fields from Gravity Forms
// instead of the "real" date picker.
// NOTE: change the input IDs to match the ones on your form

function rkv_datesubmit_fix ($form){
    //event start date field id is 3
    //event end date field ID is 4
    $raw_srt = $_POST['input_3'];
    $raw_end = $_POST['input_4'];

	// convert my dates
	$fix_srt = strtotime($raw_srt);
	$fix_end = strtotime($raw_end);

        //admin start date field ID is 35
        //admin end date field ID is 36
	$_POST['input_35'] = $fix_srt;
	$_POST['input_36'] = $fix_end;
}

add_action('gform_pre_submission', 'rkv_datesubmit_fix');