Arduino stops logging data after ~32761 seconds

Question

I made a small set up to log temperature on SD card, everything is running smoothly until it reach 9:06:01 of logging that is 32761 seconds. I know that corresponds roughly to the maximum value of an integer. But I made all my variables unsigned long. Here is the code in the loop and the if condition to log or not. The timer is computed with currentTime (unsigned long) and startTime (unsigned long). So even if logCount*freq are both integers, they should reach maximum value after 32761 seconds, then the if condition should always be true.

void loop() { 
 String date;
 unsigned long timer;
 int freq = 60;

 sensors.requestTemperatures();
 tempC1 = printTemperature(insideThermometer, 18);
 tempC2 = printTemperature(outsideThermometer, 19);

 // start serial port
 Serial.begin(9600);

 mainMenuDraw();
 operateMainMenu();

 CurrentTime = millis();
 timer=(CurrentTime - StartTime)/1000;
 date = TimeShowFormatted(CurrentTime - StartTime);

 if (timer > logCount * freq and StopStatus == 0) {
  logCount += 1;
  LastLoggedDate=date;
  DataLogg(date);

 }
}

Answer 1

Instead of comparing absolute times you should use the following:

unsigned long now = millis();

if(now - time_last_log >= freq){
    logCount += 1;
    LastLoggedDate=date;
    DataLogg(date);
    time_last_log = now; 
    // or if you really want fixed intervals
    // time_last_log += freq; 
}

This is guaranteed to be overflow proof.