diff --git a/ChangeLog b/ChangeLog
index 362cbe4df..f1f245883 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,8 +1,20 @@
+2024-12-31 Richard Frith-Macdonald <rfm@gnu.org>
+
+	* Source/NSTimeZone.m: Add GNUSTEP_BUILTIN_TZ environment variable to
+	force the use of the IANA standard based time zone files we provide
+	in the distribution rather than the system supplied time zone data.
+	This allows us to standardise test behavior across different systems.
+	* Tests/base/NSCalendarDate/TestInfo: set GNUSTEP_BUILTIN_TZ=1
+	* Tests/base/NSCalendarDate/test02.m: Change from using the MET zone
+	to using CET for tests since the IANA data has changed to make MET
+	behave aas CET.
+	* Tests/base/NSTimeZone/TestInfo: set GNUSTEP_BUILTIN_TZ=1
+
 2024-11-29 Richard Frith-Macdonald <rfm@gnu.org>
 
 	* Source/NSURLProtocol.m: fix leaks due to retain loops.
 	* Source/Additions/NSObject+GNUstepBase.m: fix issues in new
-	-trackOownership mechanism to log tracked instances at exist and to
+	-trackOwnership mechanism to log tracked instances at exist and to
 	handle cases where we track instances of a class and instances of
 	its subclasses.
 
diff --git a/Source/NSTimeZone.m b/Source/NSTimeZone.m
index 6eea6fd01..5ac959bcf 100644
--- a/Source/NSTimeZone.m
+++ b/Source/NSTimeZone.m
@@ -33,6 +33,8 @@
    Time zone names can be different from system to system, but usually
    the user has already set up his timezone independant of GNUstep, so we
    should respect that information.
+   For testing purposes, the GNUSTEP_BUILTIN_TZ environment variable can
+   be set to force the system independent data to be used.
 
    We do not use a dictionary for storing time zones, since such a
    dictionary would be VERY large (~500K).  And we would have to use a
@@ -2500,17 +2502,23 @@ + (NSString*) _getTimeZoneFile: (NSString *)name
 	}
       GS_MUTEX_UNLOCK(zone_mutex);
     }
-  /* Use the system zone info if possible, otherwise, use our installed
-     info.  */
-  if (tzdir && [[NSFileManager defaultManager] fileExistsAtPath:
-    [tzdir stringByAppendingPathComponent: name] isDirectory: &isDir] == YES
-    && isDir == NO)
+
+  if (NO == [[[[NSProcessInfo processInfo] environment]
+    objectForKey: @"GNUSTEP_BUILTIN_TZ"] boolValue])
     {
-      dir = tzdir;
+      /* Use the system zone info if possible, otherwise, use our installed
+       * info.
+       */
+      if (tzdir && [[NSFileManager defaultManager] fileExistsAtPath:
+	[tzdir stringByAppendingPathComponent: name] isDirectory: &isDir]
+	&& isDir == NO)
+	{
+	  dir = tzdir;
+	}
     }
   if (dir == nil)
     {
-      dir = _time_zone_path (ZONES_DIR, nil);
+      dir = _time_zone_path(ZONES_DIR, nil);
     }
   return [dir stringByAppendingPathComponent: name];
 }
@@ -3065,7 +3073,7 @@ - (id) initWithName: (NSString*)name data: (NSData*)data
 
   NS_DURING
     {
-      size_t		nread;
+      size_t			nread;
       union input_buffer  	*up;
 
       lsp = malloc(sizeof(*lsp));
diff --git a/Tests/base/NSCalendarDate/TestInfo b/Tests/base/NSCalendarDate/TestInfo
index e69de29bb..4e74c9fea 100644
--- a/Tests/base/NSCalendarDate/TestInfo
+++ b/Tests/base/NSCalendarDate/TestInfo
@@ -0,0 +1,2 @@
+# On unix systems force testing using our inbuilt time zone data.
+export GNUSTEP_BUILTIN_TZ=1
diff --git a/Tests/base/NSCalendarDate/test02.m b/Tests/base/NSCalendarDate/test02.m
index 20297a61d..40c7e03b4 100644
--- a/Tests/base/NSCalendarDate/test02.m
+++ b/Tests/base/NSCalendarDate/test02.m
@@ -73,25 +73,25 @@ int main()
   [tmpArray addObject: @"PM"];
   [myLocale setObject: tmpArray forKey: NSAMPMDesignation];
    
-  tz = [NSTimeZone timeZoneWithName: @"MET"];
-  PASS_EQUAL([tz name], @"MET", "got time zone for dates")
+  tz = [NSTimeZone timeZoneWithName: @"CET"];
+  PASS_EQUAL([tz name], @"CET", "got time zone for dates")
 
   myBirthday = [NSCalendarDate dateWithYear: 1974 
-  			 month: 11
-			   day: 20
-			  hour: 13
-			minute: 0
-			second: 0
-		      timeZone: tz];
+				      month: 11
+				        day: 20
+				       hour: 13
+				     minute: 0
+				     second: 0
+				   timeZone: tz];
   PASS_EQUAL([myBirthday timeZone], tz, "myBirthday has expected time zone")
    
   anotherDay = [NSCalendarDate dateWithYear: 1974 
-  			 month: 1
-			   day: 2
-			  hour: 3
-			minute: 0
-			second: 0
-		      timeZone: tz];
+				      month: 1
+				        day: 2
+				       hour: 3
+				     minute: 0
+				     second: 0
+				   timeZone: tz];
   PASS_EQUAL([anotherDay timeZone], tz, "anotherDay has expected time zone")
    
   PASS([[myBirthday descriptionWithCalendarFormat: @"%%" 
@@ -176,7 +176,7 @@ int main()
   
   PASS_EQUAL([myBirthday
     descriptionWithCalendarFormat: @"%Z" locale: myLocale],
-    @"MET", "%%Z format works in description")
+    @"CET", "%%Z format works in description")
   
   PASS([[myBirthday descriptionWithCalendarFormat: @"%z" 
                      locale: myLocale] isEqualToString: @"+0100"],
@@ -200,7 +200,7 @@ int main()
   
   PASS_EQUAL([myBirthday
     descriptionWithCalendarFormat: @"%H:%M (%Z)" locale: myLocale],
-    @"13:00 (MET)", "%%H%%M format works in description")
+    @"13:00 (CET)", "%%H%%M format works in description")
   
   PASS([[myBirthday descriptionWithCalendarFormat: @"%R" 
                      locale: myLocale] isEqualToString: @"13:00"],
diff --git a/Tests/base/NSTimeZone/TestInfo b/Tests/base/NSTimeZone/TestInfo
index e69de29bb..0b9b8df48 100644
--- a/Tests/base/NSTimeZone/TestInfo
+++ b/Tests/base/NSTimeZone/TestInfo
@@ -0,0 +1,3 @@
+# On unix systems force testing using our inbuilt time zone data.
+export GNUSTEP_BUILTIN_TZ=1
+