Execute code within given time range

mycron.php

<?php
$y = date('Y');
$m = date('m');
$d = date('d');

$restrictStart = mktime(0, 30, 0, $m, $d, $y);
$restrictEnd = mktime(1, 30, 0, $m, $d, $y);
$time = time();
if ($time < $restrictStart || $time > $restrictEnd) {
	die('not yet!');
}

//Code to execute here

You can set up a cron job that runs every minute, for example, and code will make sure to execute only when time between 00:30 and 01:30.