prorated dates question

Hello,

I am trying to populate the value in the field based on the value entered, in this case date, dynamically. I am trying to calculate prorated dates till the end of the month and based on that to calculate the price.
The problem is, I don't seem to find the way how to make the value appear without the page refreshing. Any workarounds?
I appreciate any help you can give me.
Thanks,
Yulia

Here is the code I am starting of:

add_filter('gform_field_value_prorated_days', 'get_prorated_days');

function get_prorated_days($form){

    $start_date = $_POST["input_5"]; //NOTE: replace 2 with your actual start date input ID
    $arr = explode('/', $start_date);
    //$end_date = date('%m/%t/%Y', strtotime($arr[0].'/01/'.$arr[2]));  //
    $ldom = date('%t', strtotime($arr[0].'/01/'.$arr[2]));
    $end_date = $arr[0]."$ldom".$arr[2];
    $days = gf_date_diff($start_date, $end_date);

    $arr2 = explode('|', $_POST["input_6"]);
    $daily_rate = round($arr2[1]/$ldom);

    $pamt = floatval($daily_rate) * floatval($days);
    //$_POST["input_17"] = floatval($daily_rate) * floatval($days);
    return $pamt;
}

function gf_date_diff($start, $end) {
    $start_ts = strtotime($start);
    $end_ts = strtotime($end);
    $diff = $end_ts - $start_ts;
    return round($diff / 86400);
}

Correct. I need to display the number of days left in the month, before the form is submitted.
I don't have the form online yet. I running it on test server.