String dateTimeString;
bool loggingOn = false;   // Whether logging is turned on
int sampleCount = 0;      // How many samples have been logged

void checkLogStart(){
    if (selBtn && !loggingOn && FileOK) { 
        loggingOn = true;
        sampleCount = 0;
      
        // put NTP write and read here
        myFile = SD.open("datalog.txt", FILE_WRITE);
        Serial.println(F("\nDatalog file open"));
      
        // Starting timestamp
        sendNTPpacket(); 
        delay(1000);
        dateTimeString = receiveNTPpacket();
        myFile.print("Start == ");
        myFile.println(dateTimeString);
    } 
}

void logData(String output){
        if (loggingOn) {
        // save last data sample
        myFile.println(output);
        
        // increment sample count
        sampleCount = sampleCount + 1;
        if((sampleCount % 20) == 0){
          Serial.print(".");
        }
        // see if we're done
        if (sampleCount > (NUMOFSAMPLES - 1)) { 
            loggingOn = false;
            
            // Stopping Timestamp
            sendNTPpacket(); 
            delay(1000);
            dateTimeString = receiveNTPpacket();
            myFile.print("Stop == ");
            myFile.println(dateTimeString);
            
            // Close File
            myFile.close();
            Serial.println(F("\nDatalog file closed"));
        }
    }
}