NTPClient has now a method boolean isTimeValid() which indicates whether the last update was successful or not. Clock will blink if it wasn't successful.

include/NTPClient.h

@@ -26,6 +26,8 @@ class NTPClient {
 
     void          sendNTPPacket();
 
+    boolean       _timeValid      = false;
+
   public:
     NTPClient(UDP& udp);
     NTPClient(UDP& udp, long timeOffset);
@@ -69,6 +71,7 @@ class NTPClient {
     int getHours() const;
     int getMinutes() const;
     int getSeconds() const;
+    boolean getTimeValid() const;
 
     /**
      * Changes the time offset. Useful for changing timezones dynamically

src/NTPClient.cpp

@@ -66,6 +66,9 @@ bool NTPClient::forceUpdate() {
     Serial.println("Update from NTP Server");
   #endif
 
+  while(this->_udp->parsePacket() != 0)
+    this->_udp->flush();
+
   this->sendNTPPacket();
 
   // Wait till data is there or timeout...
@@ -74,10 +77,13 @@ bool NTPClient::forceUpdate() {
   do {
     delay ( 10 );
     cb = this->_udp->parsePacket();
-    if (timeout > 100) return false; // timeout after 1000 ms
+    if (timeout > 100) {
+        this->_timeValid = false;
+        return false; // timeout after 1000 ms
+    }
     timeout++;
   } while (cb == 0);
-
+  this->_timeValid = true;
   this->_lastUpdate = millis() - (10 * (timeout + 1)); // Account for delay in reading the time
 
   this->_udp->read(this->_packetBuffer, NTP_PACKET_SIZE);

src/effect_clock.cpp

@@ -24,6 +24,10 @@ void ClockEffect::loop(boolean invert, CRGB fg_color, CRGB bg_color) {
     }
     fg_color = bg_color;
   }
+  if (ntpClient.getTimeValid()==false && ntpClient.getSeconds() & 1==0) {
+	  clear(window, bg_color);
+	  return;
+  }
   int h = ntpClient.getHours();
   drawDigit(window, numbers3x5, 3, 5, 0, 1, h / 10, fg_color, invert);
   drawDigit(window, numbers3x5, 3, 5, 4, 1, h % 10, fg_color, invert);