convert gravity form date and time inputs to unix datetime

gform_unixdatetime.php

<?php 
/*
 * Takes a gravity form's date field & converts it to unix date/time format by populating a hidden field
 * 
 */

add_action('gform_pre_submission','s25_format_combstartdate');
function s25_format_combstartdate($form){
    //submission form uid is 1
    if($form["id"] != 1)
        return;
    //start date field id is 3
    $date_start = $_POST["input_3"];
    //start time field id is 2
    $time_start = $_POST["input_2"];

    $hour_start = str_pad($time_start[0], 2, "0", STR_PAD_LEFT); //making sure hour has 2 digits
    $minute_start = str_pad($time_start[1], 2, "0", STR_PAD_LEFT); //making sure minutes has 2 digits
    
    //formatting unix datetime
    $raw_start_date = "{$date_start} {$hour_start}:{$minute_start} {$time_start[2]}";
    $formatted_start_date = gmdate("Y-m-d H:i:s", strtotime($raw_start_date));
    //hidden field for combined start date-time uid is 19
    $_POST["input_19"] = $formatted_start_date;
}

?>