diff --git a/hash.c b/hash.c
index 6b3f844095..0b3745cf6e 100644
--- a/hash.c
+++ b/hash.c
@@ -4711,6 +4711,17 @@ env_delete(VALUE name)
 
     nam = env_name(name);
     val = getenv(nam);
+
+    /*
+     * ENV['TZ'] = nil has a special meaning.
+     * TZ is no longer considered up-to-date and ruby call tzset() as needed.
+     * It could be useful if sysadmin change /etc/localtime.
+     * This hack might works only on Linux glibc.
+     */
+    if (ENVMATCH(nam, TZ_ENV)) {
+        ruby_tz_uptodate_p = FALSE;
+    }
+
     if (val) {
 	VALUE value = env_str_new2(val);
 
@@ -4718,9 +4729,6 @@ env_delete(VALUE name)
 	if (ENVMATCH(nam, PATH_ENV)) {
 	    RB_GC_GUARD(name);
 	}
-	else if (ENVMATCH(nam, TZ_ENV)) {
-	    ruby_tz_uptodate_p = false;
-	}
 	return value;
     }
     return Qnil;
diff --git a/time.c b/time.c
index 3cb5a060f9..137d5e0f9f 100644
--- a/time.c
+++ b/time.c
@@ -3139,15 +3139,10 @@ find_time_t(struct tm *tptr, int utc_p, time_t *tp)
 
     find_dst = 0 < tptr->tm_isdst;
 
-#if defined(HAVE_MKTIME)
-    tm0 = *tptr;
-    if (!utc_p && (guess = mktime(&tm0)) != -1) {
-        tm = GUESS(&guess);
-        if (tm && tmcmp(tptr, tm) == 0) {
-            goto found;
-        }
+    /* /etc/localtime might be changed. reload it. */
+    if (!ruby_tz_uptodate_p) {
+        tzset();
     }
-#endif
 
     tm0 = *tptr;
     if (tm0.tm_mon < 0) {