use PHP 5.2+ DateTimeZone objects if available to get active daylight state

Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>

blocks/SG_iCal_VTimeZone.php

@@ -65,6 +65,16 @@ class SG_iCal_VTimeZone {
 	 * @return string standard|daylight
 	 */
 	private function getActive( $ts ) {
+		
+		if (class_exists('DateTimeZone')) {
+			
+			//PHP >= 5.2
+			$tz = new DateTimeZone( $this->tzid );
+			$date = new DateTime("@$ts", $tz);
+			return ($date->format('%I') == 1) ? 'daylight' : 'standard';
+			
+		} else {
+			
 			if( isset($this->cache[$ts]) ) {
 				return $this->cache[$ts];
 			}
@@ -82,3 +92,4 @@ class SG_iCal_VTimeZone {
 			return $this->cache[$ts];
 		}
 	}
+}