Date validation throw exception if the end-user enter custom values for HTTP_ACCEPT_LANGUAGE which are used internally in Zend_Locale. Those values are 'root', 'auto' and 'browser'.

I believe those internal values should never be accepted from end-user. However I can't wrap my mind around Zend_Locale well enough to provide a patch for that.

Here is a test script to be used via CLI (so we can manually set HTTP_ACCEPT_LANGUAGE):

<?php
require_once('Zend/Version.php');
require_once('Zend/Locale.php');
require_once('Zend/Registry.php');
require_once('Zend/Validate/Date.php');

if (php_sapi_name() != 'cli')
    die('must run as CLI script');

// Extends Zend_Locale, so we can reset its state between tests
class Zend_LocaleReset extends Zend_Locale {
    public static function reset() {
        self::$_auto = null;
        self::$_browser = null;
        self::$_environment = null;
    }
}

function test($accept) 
{
    Zend_LocaleReset::reset();
    $_SERVER['HTTP_ACCEPT_LANGUAGE'] = $accept;

    $locale = new Zend_Locale();
    Zend_Registry::set('Zend_Locale', $locale);
    
    echo $accept . "\t=>\t" . $locale->getLanguage() . ': ';
    try {
        $validator = new Zend_Validate_Date(array('format' => 'Y-m-d'));
        $validator->isValid('2012-08-08');
        echo 'OK';
    } catch (Exception $exc) {
        echo $exc->getMessage();
    }
    echo PHP_EOL;
    
}

echo 'Zend Version: ' . Zend_Version::VERSION . PHP_EOL . PHP_EOL;
test(null);
test('');
test('en');
test('en-US,en;q=0.8,ko;q=0.6');
test('fr');
test('non valid string');
test('root');
test('auto');
test('browser');
test('environment');

It will output the following:

$ php test.php 
Zend Version: 1.11.0dev

    =>  en: OK
    =>  en: OK
en  =>  en: OK
en-US,en;q=0.8,ko;q=0.6 =>  en: OK
fr  =>  fr: OK
non valid string    =>  en: OK
root    =>  root: The locale '' is no known locale
auto    =>  root: The locale '' is no known locale
browser =>  root: The locale '' is no known locale
environment =>  en: OK