Index: generic/tclClock.c
==================================================================
--- generic/tclClock.c
+++ generic/tclClock.c
@@ -2005,25 +2005,28 @@
 static void
 TzsetIfNecessary(void)
 {
     static char *tzWas = NULL;	/* Previous value of TZ, protected by
 				 * clockMutex. */
+    static char first = 0;	/* First time? */
     const char *tzIsNow;	/* Current value of TZ */
 
     Tcl_MutexLock(&clockMutex);
     tzIsNow = getenv("TZ");
-    if (tzIsNow != NULL && (tzWas == NULL || strcmp(tzIsNow, tzWas) != 0)) {
+    int mod = 	((size_t)tzWas ^ (size_t)tzIsNow) || 
+		((size_t)tzWas & (size_t)tzIsNow && strcmp(tzIsNow, tzWas) != 0);
+    if (first || mod) {
+	first = 0;
 	tzset();
 	if (tzWas != NULL) {
 	    ckfree(tzWas);
+	    tzWas = NULL;
 	}
-	tzWas = ckalloc(strlen(tzIsNow) + 1);
-	strcpy(tzWas, tzIsNow);
-    } else if (tzIsNow == NULL && tzWas != NULL) {
-	tzset();
-	ckfree(tzWas);
-	tzWas = NULL;
+	if (tzIsNow != NULL) {
+	    tzWas = ckalloc(strlen(tzIsNow) + 1);
+	    strcpy(tzWas, tzIsNow);
+	}
     }
     Tcl_MutexUnlock(&clockMutex);
 }
 
 /*