fix: fixes issues with Moscow winter time

Instead of using the internal timezone when converting a string value to
a DateTime value, use UTC. This solves issues with Moscow winter time,
while allowing existing tests to continue passing, and is more
equivalent to the custom week format support, which does not use a
timezone.

Fixes #257

Author: weierophinney

src/DateStep.php

@@ -166,10 +166,10 @@ protected function convertString($value, $addErrors = true)
         if (strpos($this->format, 'Y-\WW') === 0
             && preg_match('/^([0-9]{4})\-W([0-9]{2})/', $value, $matches)
         ) {
-            $date = new DateTime;
+            $date = new DateTime();
             $date->setISODate($matches[1], $matches[2]);
         } else {
-            $date = DateTime::createFromFormat($this->format, $value, $this->timezone);
+            $date = DateTime::createFromFormat($this->format, $value, new DateTimeZone('UTC'));
         }
 
         // Invalid dates can show up as warnings (ie. "2007-02-99")