<?php
	for ($i = 0; $i <= 1440; $i+=15) : 
	$date = date("g:i A",mktime(0,$i,0,1,1,2000));
?>
<select>
	<option value="<?php echo $date; ?>"><?php echo $date; ?></option>
</select>
<?php
	endfor;
?>

Outputs a select box that has timestamps from 12AM-12PM in 15min increments.

Timestamp Selectbox 15 min intervals

PHP