diff --git a/src/NTPClientLib.cpp b/src/NTPClientLib.cpp
index 00b809c..b122b6e 100644
--- a/src/NTPClientLib.cpp
+++ b/src/NTPClientLib.cpp
@@ -612,7 +612,17 @@ void NTPClient::onNTPSyncEvent (onSyncEvent_t handler) {
 }
 
 time_t NTPClient::getUptime () {
-    _uptime = _uptime + (millis () - _uptime);
+    unsigned long long currentMillis = millis();
+    if (currentMillis < _lastMillis)
+    {
+        // millis() has rolled over
+        _uptime += (ULONG_MAX - _lastMillis) + currentMillis;
+    }
+    else
+    {
+        _uptime += (currentMillis - _lastMillis);
+    }
+    _lastMillis = currentMillis;
     return _uptime / 1000;
 }
 
diff --git a/src/NtpClientLib.h b/src/NtpClientLib.h
index 2d07631..549f7d1 100644
--- a/src/NtpClientLib.h
+++ b/src/NtpClientLib.h
@@ -452,7 +452,8 @@ class NTPClient {
     int _longInterval = DEFAULT_NTP_INTERVAL;          ///< Interval to set periodic time sync
     time_t _lastSyncd = 0;      ///< Stored time of last successful sync
     time_t _firstSync = 0;      ///< Stored time of first successful sync after boot
-    unsigned long _uptime = 0;  ///< Time since boot
+    unsigned long long _uptime = 0;  ///< Time since boot
+    unsigned long _lastMillis = 0;        ///< defined to avoid millis() rollover
     uint16_t ntpTimeout = 1500; ///< Response timeout for NTP requests
     onSyncEvent_t onSyncEvent;  ///< Event handler callback