Example of using PHP's Date objects to generate a select element.

select.php

<select name="time" id="time">
<?php
        $now = new DateTime();
        $select = new DateTime();
        $select->setTime(0,0);
        $tomorrow = new DateTime('tomorrow');
        $tomorrow->setTime(0,0);
?>
        <?php while($select < $tomorrow): ?>
        <?php $diff = $select->diff($now) ?>
        <option value='<?php echo $select->format('g:i a')?>'<?php if((0 == $diff->h) AND ($diff->i <= 15)):?>selected<?php endif;?> ><?php echo $select->format('g:i a')?></option>
        <?php $select->add(new DateInterval('PT30M')); ?>

        <?php endwhile; ?>
</select>