Accounting for fractional seconds

- Optionally collect the fractional part of the NTP response, calculate the millisecond
  offset that it indicates and use that in subsequent epoch values.
- While we do not return time with more precision than one second, the time returned
  is still more accurate.

Author: chrisy

NTPClient.cpp

@@ -112,6 +112,18 @@ bool NTPClient::forceUpdate() {
   // this is NTP time (seconds since Jan 1 1900):
   unsigned long secsSince1900 = highWord << 16 | lowWord;
 
+  if (this->_fractionalTime) {
+    highWord = word(this->_packetBuffer[44], this->_packetBuffer[45]);
+    lowWord = word(this->_packetBuffer[46], this->_packetBuffer[47]);
+
+    unsigned long fracSecs = highWord << 16 | lowWord;
+
+    // Convert the fractional part into ms
+    this->_fracOffset = (unsigned long)(((double)fracSecs / 4294967295.0) * 1000.0);
+  } else {
+    this->_fracOffset = 0;
+  }
+
   this->_currentEpoc = secsSince1900 - SEVENZYYEARS;
 
   return true;  // return true after successful update
@@ -129,7 +141,7 @@ bool NTPClient::update() {
 unsigned long NTPClient::getEpochTime() const {
   return this->_timeOffset + // User offset
          this->_currentEpoc + // Epoc returned by the NTP server
-         ((millis() - this->_lastUpdate) / 1000); // Time since last update
+         ((this->_fracOffset + millis() - this->_lastUpdate) / 1000); // Time since last update
 }
 
 int NTPClient::getDay() const {
@@ -173,6 +185,10 @@ void NTPClient::setUpdateInterval(unsigned long updateInterval) {
   this->_updateInterval = updateInterval;
 }
 
+void NTPClient::setFractionalTime(bool enable) {
+  this->_fractionalTime = enable;
+}
+
 void NTPClient::setPoolServerName(const char* poolServerName) {
     this->_poolServerName = poolServerName;
 }

NTPClient.h

@@ -17,11 +17,13 @@ class NTPClient {
     IPAddress     _poolServerIP;
     int           _port           = NTP_DEFAULT_LOCAL_PORT;
     long          _timeOffset     = 0;
+    bool          _fractionalTime = false;
 
     unsigned long _updateInterval = 60000;  // In ms
 
     unsigned long _currentEpoc    = 0;      // In s
     unsigned long _lastUpdate     = 0;      // In ms
+    unsigned long _fracOffset     = 0;      // In ms
 
     byte          _packetBuffer[NTP_PACKET_SIZE];
 
@@ -85,6 +87,12 @@ class NTPClient {
      */
     void setUpdateInterval(unsigned long updateInterval);
 
+    /** Enable or disable tracking fractional seconds from the NTP response.
+     * We don't return time with that precision, but we do account for it when
+     * working out what the time now is.
+     */
+    void setFractionalTime(bool enable);
+
     /**
      * @return time formatted like `hh:mm:ss`
      */