Added processing of fractional time in forceUpdate() to allow syncing to the exact NTP time.

Added sanity checking of received timestamp before using it.
Added millis() rollover protection in getEpochTime().
Added new function, setEpochTime(), to set NTPClient's internal currentEpoch from hardware RTC time.
Changed the default updateInterval to 1 hour.
Added 'Advanced_with_RTC' example to demonstrate new capabilities and accuracy.

NTPClient.cpp

@@ -102,8 +102,6 @@ bool NTPClient::forceUpdate() {
     timeout++;
   } while (cb == 0);
 
-  this->_lastUpdate = millis() - (10 * (timeout + 1)); // Account for delay in reading the time
-
   this->_udp->read(this->_packetBuffer, NTP_PACKET_SIZE);
 
   unsigned long highWord = word(this->_packetBuffer[40], this->_packetBuffer[41]);
@@ -111,10 +109,20 @@ bool NTPClient::forceUpdate() {
   // combine the four bytes (two words) into a long integer
   // this is NTP time (seconds since Jan 1 1900):
   unsigned long secsSince1900 = highWord << 16 | lowWord;
+  /// return early if received timestamp is invalid
+  if (secsSince1900 == 0) return false;
   
-  this->_currentEpoc = secsSince1900 - SEVENZYYEARS;
+  unsigned long unixEpoch = secsSince1900 - SEVENZYYEARS;
+  /// compute the fractional time. we will use it to sync to the nearest millis() 
+  highWord = word(this->_packetBuffer[44], this->_packetBuffer[45]);
+  lowWord = word(this->_packetBuffer[46], this->_packetBuffer[47]);
+  unsigned long fractionPart = highWord << 16 | lowWord;
+  unsigned long milliseconds = ((unsigned long long) fractionPart * 1000) >> 32;
   
-  return true;  // return true after successful update
+  this->_lastUpdate = (unsigned long) (millis() - milliseconds);
+  this->_currentEpoc = unixEpoch;
+    
+  return true;
 }
 
 bool NTPClient::update() {
@@ -131,9 +139,24 @@ bool NTPClient::isTimeSet() const {
 }
 
 unsigned long NTPClient::getEpochTime() const {
+  unsigned long t = millis();
+  /// if millis() has rollover, get difference between lastUpdate and maximum value of uint32_t.
+  /// then add it to current value of millis() and add 1 for when millis() was 0.
+  if (t < this->_lastUpdate) {
+    t = (((unsigned long) -1) - this->_lastUpdate) + t + 1; 
+  } else {
+    t = t - this->_lastUpdate;   
+  }
   return this->_timeOffset + // User offset
-         this->_currentEpoc + // Epoch returned by the NTP server
-         ((millis() - this->_lastUpdate) / 1000); // Time since last update
+         this->_currentEpoc + // Epoc returned by the NTP server
+         (t / 1000); // Time since last update
+}
+
+unsigned long NTPClient::setEpochTime(unsigned long localTime) {
+    this->_currentEpoc = localTime - this->_timeOffset;
+    this->_lastUpdate = millis();
+    /// Return Unix epoch
+    return (this->_currentEpoc);
 }
 
 int NTPClient::getDay() const {

NTPClient.h

@@ -18,7 +18,7 @@ class NTPClient {
     unsigned int  _port           = NTP_DEFAULT_LOCAL_PORT;
     long          _timeOffset     = 0;
 
-    unsigned long _updateInterval = 60000;  // In ms
+    unsigned long _updateInterval = 3600000;  // In ms
 
     unsigned long _currentEpoc    = 0;      // In s
     unsigned long _lastUpdate     = 0;      // In ms
@@ -107,6 +107,14 @@ class NTPClient {
      */
     unsigned long getEpochTime() const;
 
+    /**
+     * Set internal _currentEpoch from the localTime.  Useful when a hardware
+     * RTC is used to set the initial unix time of NTPClient.
+     * @param localTime, current local time in seconds since Jan. 1, 1970
+     * @return unix epoch in seconds (GMT) since Jan. 1, 1970
+     */
+    unsigned long setEpochTime(unsigned long localTime);
+
     /**
      * Stops the underlying UDP client
      */

examples/Advanced_with_RTC/Advanced_with_RTC.ino

@@ -0,0 +1,91 @@
+/* Advanced NTPClient With RTC example 
+ *  Copyright (c) 2021 by Anthony F. Dela Paz
+ *  
+ * Featuring:
+ *  + an RTC (RTCLib can use hardware RTC by simply changing 'RTC_Millis' with appropriate declaration).
+ *  + DateTime facility of the RTCLib to facilitate epochtime conversion
+ *  + separately setting the ntp server, timezone offset, and update interval
+ *  + syncing the internal software RTC of the NTPClient with the RTC time
+ *  
+ * This example code demonstrates NTPClient's new capability of syncing with NTP time server
+ * down to millisecond accuracy.  It's best to upload this code to at least two ESP-12E modules
+ * to see how both their blue LED blinks simultaneously upon synchronization with NTP server.
+ *  
+ * The MIT License (MIT) 
+ * 
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#include <ESP8266WiFi.h>
+#include <WiFiUdp.h>
+#include <NTPClient.h>
+#include "RTClib.h"
+
+const char *ssid     = "<SSID>"; // change the SSID and PASSWORD
+const char *password = "<PASSWORD>";
+
+WiFiUDP ntpUDP;
+NTPClient timeClient(ntpUDP);
+
+RTC_Millis rtc; // this should be a hardware RTC but that won't work as a quick example
+
+const byte ledBlue = 2;  // Blue LED on the ESP-12E module
+unsigned long oldTick, blinkMillis;
+
+void setup() {
+
+  pinMode(ledBlue, OUTPUT);
+
+  Serial.begin(115200);
+
+  WiFi.begin(ssid, password);
+
+  while ( WiFi.status() != WL_CONNECTED ) {
+    delay ( 500 );
+    Serial.print ( "." );
+  }
+
+  // following line sets the RTC to the date & time this sketch was compiled
+  rtc.begin(DateTime(F(__DATE__), F(__TIME__)));
+  // This line sets the RTC with an explicit local date & time, for example to set
+  // June 9, 2021 at 12:52:36 PM, you would call:
+  // rtc.adjust(DateTime(2021, 6, 9, 12, 52, 30));
+
+  timeClient.begin();
+  timeClient.setTimeOffset((-4) * 3600); // in seconds, Time zone in New York, NY, USA (GMT-4)
+  timeClient.setPoolServerName("europe.pool.ntp.org");
+  timeClient.setUpdateInterval(3600); // seconds
+  timeClient.setEpochTime(rtc.now().unixtime()); // set the timeClient's internal software rtc time.
+}
+
+void loop() {
+  timeClient.update();
+
+  // Synchronize the blinking LED with transition of seconds.
+  if (timeClient.getEpochTime() != oldTick) {
+    oldTick = timeClient.getEpochTime();
+    blinkMillis = millis();
+    Serial.println(timeClient.getFormattedTime());
+  }
+
+  // blink the LED once per second with 500ms ON-time
+  if ((millis() - blinkMillis) % 1000 > 500) {
+    digitalWrite(ledBlue, HIGH);
+  } else {
+    digitalWrite(ledBlue, LOW);
+  }
+}

keywords.txt

@@ -22,3 +22,4 @@ getEpochTime	KEYWORD2
 setTimeOffset	KEYWORD2
 setUpdateInterval	KEYWORD2
 setPoolServerName	KEYWORD2
+setEpochTime    KEYWORD2