diff --git a/libraries/AP_RTC/AP_RTC.cpp b/libraries/AP_RTC/AP_RTC.cpp
index c3e978d3fb..781e1602fb 100644
--- a/libraries/AP_RTC/AP_RTC.cpp
+++ b/libraries/AP_RTC/AP_RTC.cpp
@@ -63,6 +63,7 @@ void AP_RTC::set_utc_usec(uint64_t time_utc_usec, source_type type)
         // can't allow time to go backwards, ever
         return;
     }
+    WITH_SEMAPHORE(rsem);
 
     rtc_shift = tmp;
 
diff --git a/libraries/AP_RTC/AP_RTC.h b/libraries/AP_RTC/AP_RTC.h
index 373cc6b62d..d0c9f79a37 100644
--- a/libraries/AP_RTC/AP_RTC.h
+++ b/libraries/AP_RTC/AP_RTC.h
@@ -49,9 +49,15 @@ public:
         return _singleton;
     }
 
+    // allow threads to lock against RTC update
+    HAL_Semaphore &get_semaphore(void) {
+        return rsem;
+    }
+    
 private:
 
     static AP_RTC *_singleton;
+    HAL_Semaphore_Recursive rsem;
 
     source_type rtc_source_type = SOURCE_NONE;
     int64_t rtc_shift;