see the php date manual page:
function.date.php
either use a capitol H to return the hour in 24 hour format, add a lower case "a" to output Lowercase am / pm at the end, or add an upper case "A" for upper case AM / PM
<?php echo (date('Y-m-d h:i:s a')); ?>
would output: 2011-07-13 05:29:48 pm
<?php echo (date('Y-m-d h:i:s A')); ?>
would output: 2011-07-13 05:29:48 PM
<?php echo (date('Y-m-d H:i:s')); ?>
would output: 2011-07-13 17:29:48