Description:
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DateTime objects created with month names that have any letter appended (other than 'j', 'J', 'I', 'V', or 'X'), and fall between the years 2000 and 2059, default to the current year rather than showing the specified year or throwing an error.

The following strings result in incorrect years:
'9 Junn 2000' -> 9 Jun 2013
'9 XZ 2014' -> 9 Oct 2013
'9 Decemberi 2020' -> 9 Dec 2013

However, both of the following give the correct year:
'9 Decemberi 1999' -> 9 Dec 1999
'9 Decemberi 2060' -> 9 Dec 2060

And all of the following throw an error (which seems sensible):
'9 Decemberj 1999' -> Error
'9 DECJ 2060' -> Error
'9 DECI 2060' -> Error
'9 DECX 2060' -> Error

PHP seems to interpret an appended 'j' or 'J' as a timezone indicator (?!), and 'I', 'V' and 'X' are valid Roman month names, so they trigger a duplicate month error.

The mystery is why PHP doesn't error out in the other cases (and why the years 2000 to 2059 affect the interpretation differently).

This bug blocks the following MediaWiki bug: https://bugzilla.wikimedia.org/show_bug.cgi?id=33579

Test script:
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$tz = new DateTimeZone("Europe/Amsterdam");
$dateObject = new DateTime( '9 junn 2000', $tz );
echo $dateObject->format( 'Y' );
$dateObject = new DateTime( '9 XZ 2014', $tz );
echo $dateObject->format( 'Y' );
$dateObject = new DateTime( '9 decemberi 2020', $tz );
echo $dateObject->format( 'Y' );

Expected result:
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Error or 2000
Error or 2014
Error or 2020

Actual result:
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2013 (current year)
2013 (current year)
2013 (current year)