diff --git a/ChangeLog b/ChangeLog
index 0755fe1c1..32a0b0957 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,8 @@
 
 	* Headers/Foundation/NSString.h: Add explicit warnings about OSX
 	compatible modifications to values returned by path methods.
+	* Source/NSTimeZone.m: Don't guess at tzname[0] if the value of
+	daylight shows it's not valid.
 
 2011-05-31  Niels Grewe <niels.grewe@halbordnung.de>
 
diff --git a/Source/NSTimeZone.m b/Source/NSTimeZone.m
index f1f8875f5..4721693cc 100644
--- a/Source/NSTimeZone.m
+++ b/Source/NSTimeZone.m
@@ -1526,13 +1526,16 @@ static NSMapTable	*absolutes = 0;
         }
 #if HAVE_TZSET
       /*
-       * Try to get timezone from tzset and tzname
+       * Try to get timezone from tzset and tzname/daylight.
+       * If daylight is non-zero, then tzname[0] is only the name
+       * the the zone for part of the year, so we can't use it as
+       * the definitive zone.
        */
       if (localZoneString == nil)
 	{
           localZoneSource = @"function: 'tzset()/tzname'";
 	  tzset();
-	  if (tzname[0] != NULL && *tzname[0] != '\0')
+	  if (NULL != tzname[0] && '\0' != *tzname[0] && 0 == daylight)
 	    localZoneString = [NSString stringWithUTF8String: tzname[0]];
 	}
 #endif