diff --git a/ChangeLog b/ChangeLog
index 571f762d7f..b81aa1d825 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Mar 27 12:02:45 2013  Tanaka Akira  <akr@fsij.org>
+
+	* internal.h (TIMET_MAX_PLUS_ONE): definition simplified.
+
 Wed Mar 27 06:39:41 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/mkmf.rb (MAIN_DOES_NOTHING): force to refer symbols for tests
diff --git a/internal.h b/internal.h
index 5211411a06..fccbeb64f7 100644
--- a/internal.h
+++ b/internal.h
@@ -22,14 +22,11 @@ extern "C" {
 #if SIGNEDNESS_OF_TIME_T < 0	/* signed */
 # define TIMET_MAX (time_t)((~(unsigned_time_t)0) >> 1)
 # define TIMET_MIN (time_t)(((unsigned_time_t)1) << (sizeof(time_t) * CHAR_BIT - 1))
-# define TIMET_MAX_PLUS_ONE \
-    (((time_t)1 << (sizeof(time_t) * CHAR_BIT / 2)) * (double)((time_t)1 << (sizeof(time_t) * CHAR_BIT / 2 - 1)))
 #elif SIGNEDNESS_OF_TIME_T > 0	/* unsigned */
 # define TIMET_MAX (time_t)(~(unsigned_time_t)0)
 # define TIMET_MIN (time_t)0
-# define TIMET_MAX_PLUS_ONE \
-    (((time_t)1 << (sizeof(time_t) * CHAR_BIT / 2)) * (double)((time_t)1 << (sizeof(time_t) * CHAR_BIT / 2))))
 #endif
+#define TIMET_MAX_PLUS_ONE (2*(double)(TIMET_MAX/2+1))
 
 struct rb_deprecated_classext_struct {
     char conflict[sizeof(VALUE) * 3];