A simple Javascript timer for Magic Coupon

my_magic_coupon_timer.php

<?php
//Add new placeholder - Place {timer} on the message
add_filter( 'magic_coupon_html_message_replace_tags', function( $replace_tags ) {
	$replace_tags['timer'] = '[my_magic_coupon_timer expire="{cookie_expire_timestamp}"]';
	return $replace_tags;
} );
//The shortcode
add_shortcode( 'my_magic_coupon_timer', function( $atts ) {
	ob_start();
	$rand = rand( 0,99999 );
	$time = intval( $atts['expire'] ) - time();
	?>
	<script type="text/javascript">
	function startTimer<?php echo $rand; ?>(duration, display) {
		var timer = duration, minutes, seconds;
		setInterval(function () {
			minutes = parseInt(timer / 60, 10)
			seconds = parseInt(timer % 60, 10);
			minutes = minutes < 10 ? "0" + minutes : minutes;
			seconds = seconds < 10 ? "0" + seconds : seconds;
			display.textContent = minutes + ":" + seconds;
			if (--timer < 0) {
				timer = duration;
			}
		}, 1000);
	}
	window.onload = function () {
		var display = document.querySelector('#time<?php echo $rand; ?>');
		startTimer<?php echo $rand; ?>(<?php echo $time; ?>, display);
	};
	</script>
	<span id="time<?php echo $rand; ?>"><?php
	$minutes = intval( $time / 60 );
	$seconds = $time - ( $minutes * 60 );
	echo ( $minutes >= 10 ? $minutes : '0'.$minutes ).':'.( $seconds >= 10 ? $seconds : '0'.$seconds );
	?></span>
	<?php
	return ob_get_clean();
} );