diff --git a/hash.c b/hash.c
index c372686f12..5f237c0c9a 100644
--- a/hash.c
+++ b/hash.c
@@ -3136,6 +3136,9 @@ env_str_new2(const char *ptr)
 
 static int env_path_tainted(const char *);
 
+static const char TZ_ENV[] = "TZ";
+extern int ruby_tz_update;
+
 static rb_encoding *
 env_encoding_for(const char *name, const char *ptr)
 {
@@ -3217,6 +3220,9 @@ env_delete(VALUE obj, VALUE name)
 	    RB_GC_GUARD(name);
 	    path_tainted = 0;
 	}
+	else if (ENVMATCH(nam, TZ_ENV)) {
+	    ruby_tz_update = 0;
+	}
 	return value;
     }
     return Qnil;
@@ -3576,6 +3582,9 @@ env_aset(VALUE obj, VALUE nm, VALUE val)
 	    path_tainted_p(value);
 	}
     }
+    else if (ENVMATCH(name, TZ_ENV)) {
+	ruby_tz_update = 0;
+    }
     return val;
 }
 
diff --git a/time.c b/time.c
index 30bb371781..d167756b18 100644
--- a/time.c
+++ b/time.c
@@ -643,12 +643,18 @@ static struct vtm *localtimew(wideval_t timew, struct vtm *result);
 static int leap_year_p(long y);
 #define leap_year_v_p(y) leap_year_p(NUM2LONG(mod((y), INT2FIX(400))))
 
+int ruby_tz_update;
+
 static struct tm *
 rb_localtime_r(const time_t *t, struct tm *result)
 {
 #if defined __APPLE__ && defined __LP64__
     if (*t != (time_t)(int)*t) return NULL;
 #endif
+    if (!ruby_tz_update) {
+	ruby_tz_update = 1;
+	tzset();
+    }
 #ifdef HAVE_GMTIME_R
     result = localtime_r(t, result);
 #else
@@ -674,7 +680,7 @@ rb_localtime_r(const time_t *t, struct tm *result)
 #endif
     return result;
 }
-#define LOCALTIME(tm, result) (tzset(),rb_localtime_r((tm), &(result)))
+#define LOCALTIME(tm, result) rb_localtime_r((tm), &(result))
 
 #ifndef HAVE_STRUCT_TM_TM_GMTOFF
 static struct tm *
diff --git a/version.h b/version.h
index ac9a5e7781..4dc154a458 100644
--- a/version.h
+++ b/version.h
@@ -1,6 +1,6 @@
 #define RUBY_VERSION "2.4.5"
 #define RUBY_RELEASE_DATE "2018-08-27"
-#define RUBY_PATCHLEVEL 317
+#define RUBY_PATCHLEVEL 318
 
 #define RUBY_RELEASE_YEAR 2018
 #define RUBY_RELEASE_MONTH 8