time.c: 1969-12-31 23:59:59 is a valid time.

But `mktime(3)` for the date returns `-1` which happen to be the
error value.

mrbgems/mruby-time/src/time.c

@@ -483,7 +483,17 @@ time_mktime(mrb_state *mrb, mrb_int ayear, mrb_int amonth, mrb_int aday,
     nowsecs = mktime(&nowtime);
   }
   if (nowsecs == (time_t)-1) {
-    mrb_raise(mrb, E_ARGUMENT_ERROR, "Not a valid time");
+    nowtime.tm_sec += 1;
+    if (timezone == MRB_TIMEZONE_UTC) {
+      nowsecs = timegm(&nowtime);
+    }
+    else {
+      nowsecs = mktime(&nowtime);
+    }
+    if (nowsecs != 0) {
+      mrb_raise(mrb, E_ARGUMENT_ERROR, "Not a valid time");
+    }
+    nowsecs = (time_t)-1;
   }
 
   return time_alloc_time(mrb, nowsecs, ausec, timezone);

mrbgems/mruby-time/test/time.rb

@@ -267,7 +267,3 @@ assert('2000 times 500us make a second') do
   end
   assert_equal(0, t.usec)
 end
-
-assert('Time.gm with Dec 31 23:59:59 1969 raise ArgumentError') do
-  assert_raise(ArgumentError) {Time.gm(1969, 12, 31, 23, 59, 59)}
-end