Description:
------------
Format returns incorrect number of digits for negative years -0001 to -0999.  See ISO 8601:2004 and http://en.wikipedia.org/wiki/Year_zero: 

The "basic" format for year 0 is the four-digit form 0000, which equals the historical year 1 BC. Several "expanded" formats are possible: -0000 and +0000, as well as five- and six-digit versions. Earlier years are also negative four-, five- or six-digit years, which have an absolute value one less than the equivalent BC year, hence -0001 = 2 BC.


Reproduce code:
---------------
<?
$date = new DateTime('2007-06-28');
$date->modify('-3006 years');
echo $date->format(DATE_ISO8601);
?>

Outputs:

-999-06-17T00:00:00-0600

Expected:

-0999-06-17T00:00:00-0600

<?
$date = new DateTime('2007-06-28');
$date->modify('-2008 years');
echo $date->format(DATE_ISO8601);
?>

Outputs:

-001-06-17T00:00:00-0600

Expected:

-0001-06-17T00:00:00-0600