Print sendtime of messsages.

Change-Id: I42579bce6133a536df61da9355aed763a0b2e614
diff --git a/aos/common/message.h b/aos/common/message.h
index 8ae7f16..f46969f 100644
--- a/aos/common/message.h
+++ b/aos/common/message.h
@@ -28,7 +28,11 @@
   size_t Serialize(char *buffer) const;
 
   // Populates sent_time with the current time.
-  void SetTimeToNow() { sent_time = monotonic_clock::now(); }
+  void SetTimeToNow() {
+    if (sent_time == monotonic_clock::min_time) {
+      sent_time = monotonic_clock::now();
+    }
+  }
 
   // Writes the contents of the message to the provided buffer.
   size_t Print(char *buffer, int length) const;