Fix #236: Prevent fabricated epoch timestamps before NTP synchronization

NTPClient.cpp

@@ -130,7 +130,27 @@ bool NTPClient::isTimeSet() const {
   return (this->_lastUpdate != 0); // returns true if the time has been set, else false
 }
 
+bool NTPClient::isTimeValid() const {
+  // First check if time has been set at all
+  if (!isTimeSet()) {
+    return false;
+  }
+
+  // Then check if the epoch is reasonable (after Jan 1, 2000)
+  // This catches edge cases where time might be "set" but invalid
+  // 946684800 = Jan 1, 2000, 00:00:00 UTC
+  unsigned long epoch = getEpochTime();
+  return (epoch > 946684800);
+}
+
 unsigned long NTPClient::getEpochTime() const {
+  // Return 0 if time has not been synchronized yet
+  // This prevents returning fabricated uptime values (millis()/1000)
+
+  if (this->_lastUpdate == 0) {
+    return 0;
+  }
+
   return this->_timeOffset + // User offset
          this->_currentEpoc + // Epoch returned by the NTP server
          ((millis() - this->_lastUpdate) / 1000); // Time since last update

NTPClient.h

@@ -81,6 +81,18 @@ class NTPClient {
      */
     bool isTimeSet() const;
 
+    /**
+     * Checks if the time is both set AND valid (reasonable epoch value).
+     * This is a stronger check than isTimeSet() - it verifies the epoch is
+     * greater than a minimum threshold (Jan 1, 2000) to catch edge cases.
+     *
+     * Recommended for security-sensitive applications that need to ensure
+     * time is not only synchronized but also plausible.
+     *
+     * @return true if time is set and epoch > 946684800 (Jan 1, 2000), else false
+     */
+    bool isTimeValid() const;
+
     int getDay() const;
     int getHours() const;
     int getMinutes() const;