diff --git a/a8/timer.cc b/a8/timer.cc
index 8aaf396..9a2585c 100644
--- a/a8/timer.cc
+++ b/a8/timer.cc
@@ -256,7 +256,7 @@ namespace a8
         if (!timer) {
             abort();
         }
-        long long remain_time = a8::XGetTickCount() - timer->expires;
+        long long remain_time = timer->expires - a8::XGetTickCount();
         return std::max(remain_time, (long long)0);
     }
 
diff --git a/a8/xtimer.cc b/a8/xtimer.cc
index 8b4dc23..3d7d692 100644
--- a/a8/xtimer.cc
+++ b/a8/xtimer.cc
@@ -267,7 +267,7 @@ namespace a8
         if (!timer) {
             abort();
         }
-        long long remain_time = get_tick_count_func_(context_) - timer->expires;
+        long long remain_time = timer->expires - get_tick_count_func_(context_);
         return std::max(remain_time, (long long)0);
     }