Compare time differences instead of timestamps (#1648)

Comparing two results of millis() can result in erroneous
behavior when the value of millis() wraps around, which
happens after around 25 days of uptime. Comparing time
difference (by subtracting two timestamps) works well, provided
timestamps are not too old (less than 25 days old).

main/ZsensorADC.ino

@@ -34,6 +34,8 @@
 ADC_MODE(ADC_TOUT);
 #  endif
 
+static int persistedadc = -1024; // so that the first reading will always be published
+
 //Time used to wait for an interval before resending adc value
 unsigned long timeadc = 0;
 unsigned long timeadcpub = 0;
@@ -42,13 +44,12 @@ void setupADC() {
 }
 
 void MeasureADC() {
-  if (millis() > (timeadc + TimeBetweenReadingADC)) { //retrieving value of temperature and humidity of the box from DHT every xUL
+  if (millis() - timeadc > TimeBetweenReadingADC) { //retrieving value of temperature and humidity of the box from DHT every xUL
 #  if defined(ESP8266)
     yield();
     analogRead(ADC_GPIO); //the reliability of the readings can be significantly improved by discarding an initial reading and then averaging the results
 #  endif
     timeadc = millis();
-    static int persistedadc;
     int val = analogRead(ADC_GPIO);
     int sum_val = val;
     if (NumberOfReadingsADC > 1) { // add extra measurement for accuracy
@@ -60,7 +61,7 @@ void MeasureADC() {
     if (isnan(val)) {
       Log.error(F("Failed to read from ADC !" CR));
     } else {
-      if (val >= persistedadc + ThresholdReadingADC || val <= persistedadc - ThresholdReadingADC || (MinTimeInSecBetweenPublishingADC > 0 && millis() > (timeadcpub + (MinTimeInSecBetweenPublishingADC * 1000UL)))) {
+      if (val >= persistedadc + ThresholdReadingADC || val <= persistedadc - ThresholdReadingADC || (MinTimeInSecBetweenPublishingADC > 0 && millis() - timeadcpub > (MinTimeInSecBetweenPublishingADC * 1000UL))) {
         timeadcpub = millis();
         Log.trace(F("Creating ADC buffer" CR));
         StaticJsonDocument<JSON_MSG_BUFFER> jsonBuffer;