In phpBB/language/en/ucp.php at line 118, we have this:

 'BOARD_DATE_FORMAT_EXPLAIN' => 'The syntax used is identical to the PHP <a href="http://www.php.net/date">date()</a> function.',

In phpBB/language/en/acp/board.php at line 47, we have this: 

'DEFAULT_DATE_FORMAT_EXPLAIN' => 'The date format is the same as the PHP <code>date</code> function.',

Note that the first provides the user a handy link to the PHP Date function page, whereas the latter does not.  This is a handy feature and I can think of no reason why line 47 in board.php should not also provide a link and read:

'DEFAULT_DATE_FORMAT_EXPLAIN' => 'The date format is the same as the PHP <a href="http://www.php.net/date">date()</a> function.',