diff --git a/src/gpsread.cpp b/src/gpsread.cpp
index 45f1b65d..650c5866 100644
--- a/src/gpsread.cpp
+++ b/src/gpsread.cpp
@@ -21,6 +21,9 @@ static uint16_t nmea_txDelay_ms =
 static uint16_t nmea_txDelay_ms = 0;
 #endif
 
+// did the last packet contain a valid time?
+bool hasValidTime = false;
+
 // initialize and configure GPS
 int gps_init(void) {
 
@@ -159,6 +162,18 @@ void gps_loop(void *pvParameters) {
         delay(2); // 2ms delay according L76 datasheet
       }
 #endif
+
+#if !TIME_SYNC_LORASERVER && !TIME_SYNC_LORAWAN
+      if (!hasValidTime) {
+        if (gpstime.isUpdated() && gpstime.isValid()) {
+          hasValidTime = true;
+          if (timeSource == _unsynced) {
+            calibrateTime();
+          }
+        }
+      }
+#endif
+
     } // if
 
     // show NMEA data in verbose mode, useful for debugging GPS, bu tvery noisy
diff --git a/src/timekeeper.cpp b/src/timekeeper.cpp
index bc9e52b5..01777be7 100644
--- a/src/timekeeper.cpp
+++ b/src/timekeeper.cpp
@@ -26,7 +26,8 @@ Ticker timesyncer;
 void timeSync() { xTaskNotify(irqHandlerTask, TIMESYNC_IRQ, eSetBits); }
 
 void calibrateTime(void) {
-
+  ESP_LOGD(TAG, "[%0.3f] calibrateTime, timeSource == %d", millis() / 1000.0,
+           timeSource);
   time_t t = 0;
   uint16_t t_msec = 0;
 
@@ -49,7 +50,9 @@ void calibrateTime(void) {
 // no RTC -> fallback to GPS time
 #if (HAS_GPS)
     t = get_gpstime(&t_msec);
-    timeSource = _gps;
+    if (t) {
+      timeSource = _gps;
+    }
 #endif
 
     setMyTime((uint32_t)t, t_msec, timeSource); // set time