Add custom field to order admin woocommerce

gistfile1.txt

<?php
/**
 * Display field value on the order edit page
 */
add_action( 'woocommerce_admin_order_data_after_billing_address', 'wdt_delivery_date_admin_order_meta', 10, 1 );
function wdt_delivery_date_admin_order_meta($order){

	global $wpdb;

	$strSelect = "SELECT meta_key FROM {$wpdb->prefix}postmeta WHERE post_id = '" . $order->get_id() . "' AND meta_key LIKE '_wdt_slot_%'";
	$arrSelect = $wpdb->get_results($strSelect);

	list($date, $slot) = explode( '_', str_replace( '_wdt_slot_' . date('Y') . '_', '', $arrSelect[0]->meta_key));

    echo '<p class="address"><strong>'.__('Delivery Date').':</strong> <br/>' . $date . ' @ ' . $slot . '</p>';

	$arrData = get_option('_woo_delivery_slots');
	$arrOptions  = $arrOptions2 = array();

	foreach ($arrData[date('Y')] as $key => $value) {
		
		$flag_useful = false;
		foreach ($value as $keyI => $valueI) {
			
			if(isset($valueI['active']) && $valueI['active'] == 'true' && isset($valueI['value']) && intval($valueI['value']) > 0) {
				$flag_useful = true;
			}
		}
		if($flag_useful) {
			$arrOptions[] = $key;
			$arrOptions2[$key] = $key;
		}
	}
	$arrOptions2 = array_reverse($arrOptions2);

	$arrSlots = array();
	foreach ($arrData[date('Y')][$date] as $key => $value) {
		
		$strCount = "SELECT count(*) used FROM {$wpdb->prefix}postmeta WHERE meta_key = '_wdt_slot_" . date('Y') . '_' . $date . "_" . $key . "' GROUP BY meta_key";
		$arrCount = $wpdb->get_results($strCount);
		
		if(isset($value['active']) && $value['active'] == 'true' && ((!isset($arrCount[0]->used) || empty($arrCount[0]->used) || $arrCount[0]->used < $value['value']) || current_user_can( 'manage_options' ) ) && $value['value'] > 0) {
			$arrSlots[$key] = $key;
		}
	}

    ?><div class="edit_address"><?php
    	echo 'Delivery Date : <select name="sel_myaccount_delivery_date" id="sel_myaccount_delivery_date">';
			foreach ($arrOptions2 as $key => $value) {
				
				echo '<option value="' . $value . '" ' . ($value == $date?' selected="selected"':'') . '>' . $value . '</option>';
			}
		echo '</select>';

		echo '<br/>Delivery Slot : <select name="sel_myaccount_delivery_slot" id="sel_myaccount_delivery_slot">';
			foreach ($arrSlots as $key => $value) {
				
				echo '<option value="' . $value . '" ' . ($value == $slot?' selected="selected"':'') . '>' . $value . '</option>';
			}
		echo '</select>';
    ?></div><?php
}