Description:
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It would appear that there is a mistake in the DATE_ISO8601 predefined constant 
for the date() function.

current output: 2011-07-22T00:00:00-0500
desired output: 2011-07-22T00:00:00-05:00 

http://www.php.net/manual/en/class.datetime.php#datetime.constants.iso8601

http://en.wikipedia.org/wiki/ISO_8601

Test script:
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As best as I can tell, there's supposed to be a semicolon between the timezone offset hours:minutes.

<?php
$d = mktime (0, 0, 0, '03', '08', '2011');
// 2011-03-08T00:00:00-0600 <-- actual, incorrect (?) result
echo date(DATE_ISO8601, $d).'<br>';

// 2011-03-08T00:00:00-0600
echo date("Y-m-d\TH:i:sO", $d).'<br>';

// 2011-03-08T00:00:00-06:00 <-- expected result
echo date("Y-m-d\TH:i:sP", $d).'<br>';

?>

So the format for DATE_ISO8601 should be "Y-m-d\TH:i:sP" (capital P at the end), not "Y-m-d\TH:i:sO" (capital O at the end).

Expected result:
----------------
// 2011-03-08T00:00:00-0600 <-- actual, incorrect (?) result
echo date(DATE_ISO8601, $d);

Actual result:
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// 2011-03-08T00:00:00-06:00 <-- expected result
echo date("Y-m-d\TH:i:sP", $d);