diff --git a/include/globals.h b/include/globals.h
index bab56cb3..f132c040 100644
--- a/include/globals.h
+++ b/include/globals.h
@@ -99,6 +99,7 @@ typedef struct {
 
 enum sendprio_t { prio_low, prio_normal, prio_high };
 enum timesource_t { _gps, _rtc, _lora, _unsynced };
+enum mutexselect_t { no_mutex, do_mutex};
 
 extern std::set<uint16_t, std::less<uint16_t>, Mallocator<uint16_t>> macs;
 extern std::array<uint64_t, 0xff>::iterator it;
diff --git a/include/rtctime.h b/include/rtctime.h
index 3429d44e..08baa9f6 100644
--- a/include/rtctime.h
+++ b/include/rtctime.h
@@ -9,7 +9,7 @@
 extern RtcDS3231<TwoWire> Rtc; // make RTC instance globally available
 
 uint8_t rtc_init(void);
-uint8_t set_rtctime(time_t t);
+uint8_t set_rtctime(time_t t, mutexselect_t mutex);
 void sync_rtctime(void);
 time_t get_rtctime(void);
 float get_rtctemp(void);
diff --git a/src/rtctime.cpp b/src/rtctime.cpp
index ceabbb20..086beb29 100644
--- a/src/rtctime.cpp
+++ b/src/rtctime.cpp
@@ -46,14 +46,15 @@ uint8_t rtc_init(void) {
 
 } // rtc_init()
 
-uint8_t set_rtctime(time_t t) { // t is UTC in seconds epoch time
-  if (I2C_MUTEX_LOCK()) {
-    Rtc.SetDateTime(RtcDateTime(t - SECS_YR_2000)); // epoch -> sec2000
+uint8_t set_rtctime(time_t t, mutexselect_t mutex) { // t is sec epoch time
+  if (!mutex || I2C_MUTEX_LOCK()) {
 #ifdef RTC_INT // sync rtc 1Hz pulse on top of second
     Rtc.SetSquareWavePin(DS3231SquareWavePin_ModeNone);  // off
     Rtc.SetSquareWavePin(DS3231SquareWavePin_ModeClock); // start
 #endif
-    I2C_MUTEX_UNLOCK();
+    Rtc.SetDateTime(RtcDateTime(t - SECS_YR_2000)); // epoch -> sec2000
+    if (mutex)
+      I2C_MUTEX_UNLOCK();
     ESP_LOGI(TAG, "RTC time synced");
     return 1; // success
   } else {