diff --git a/src/main.cpp b/src/main.cpp
index bd29c1d..c97cab7 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -88,7 +88,7 @@ struct __attribute__((packed)) sensor_readings_t {
   float temperature_min = NAN; // °C
   int battery = 0;
   int8_t rssi = 0;
-  uint32_t lastUpdate = 0;
+  time_t lastUpdate = 0;
 } sensor_readings;
 
 sensor_readings_t sensors_a4cf1211c3e4, sensors_246f28d1fa5c, sensors_246f28d1a080, sensors_246f28d1eff4;
@@ -345,7 +345,7 @@ void getSensorMeasurements() {
   ESP_LOGI(TAG, "Battery       : %5d   ", sensor_readings.battery);
   ESP_LOGI(TAG, "Heap          : %5lu", ESP.getFreeHeap());
 
-  sensor_readings.lastUpdate = millis();
+  sensor_readings.lastUpdate = getTimestamp();
 }
 
 
@@ -386,7 +386,7 @@ void receiveMqtt(const char* topic, const char* data) {
       } else if (strcmp("/voc", topic_last) == 0) {
         sensor->voc = atof(data);
       }
-      sensor->lastUpdate = millis();
+      sensor->lastUpdate = getTimestamp(); // ToDo
     }
 }
 
@@ -562,16 +562,16 @@ void displayValues() {
     // other nodes
     display.setFont(NULL);
     display.setCursor(0, y_offset+70);
-    if (!ota.getMAC().equals("246f28d1fa5c") && millis() - sensors_246f28d1fa5c.lastUpdate < 15*60*1000) {
+    if (!ota.getMAC().equals("246f28d1fa5c") && getTimestamp() - sensors_246f28d1fa5c.lastUpdate < 15*60) {
       display.printf("246f28d1fa5c: %4.1f %4.1f %6.1f\n", sensors_246f28d1fa5c.temperature, sensors_246f28d1fa5c.humidity, sensors_246f28d1fa5c.pressure);
     }
-    if (!ota.getMAC().equals("a4cf1211c3e4") && millis() - sensors_a4cf1211c3e4.lastUpdate < 15*60*1000) {
+    if (!ota.getMAC().equals("a4cf1211c3e4") && getTimestamp() - sensors_a4cf1211c3e4.lastUpdate < 15*60) {
       display.printf("a4cf1211c3e4: %4.1f %4.1f %6.1f\n", sensors_a4cf1211c3e4.temperature, sensors_a4cf1211c3e4.humidity, sensors_a4cf1211c3e4.pressure);
     }
-    if (!ota.getMAC().equals("246f28d1a080") && millis() - sensors_246f28d1a080.lastUpdate < 15*60*1000) {
+    if (!ota.getMAC().equals("246f28d1a080") && getTimestamp() - sensors_246f28d1a080.lastUpdate < 15*60) {
       display.printf("246f28d1a080: %4.1f %4.1f %6.1f\n", sensors_246f28d1a080.temperature, sensors_246f28d1a080.humidity, sensors_246f28d1a080.pressure);
     }
-    if (!ota.getMAC().equals("246f28d1eff4") && millis() - sensors_246f28d1eff4.lastUpdate < 15*60*1000) {
+    if (!ota.getMAC().equals("246f28d1eff4") && getTimestamp() - sensors_246f28d1eff4.lastUpdate < 15*60) {
       display.printf("246f28d1eff4: %4.1f %4.1f %6.1f\n", sensors_246f28d1eff4.temperature, sensors_246f28d1eff4.humidity, sensors_246f28d1eff4.pressure);
     }