Convert a date to ISO8601 date format

Use the_time or get_the_time, which accept the same format parameters as php’s date.

// assign to a variable
$iso8601_date = get_the_time('c');

// or output directly
the_time('c');

EDIT- converting formats with php:

$date = "September 11, 2011 9:00 am";
$time = strtotime( $date );
echo date( 'c', $time );