const unsigned long intervalNTP = ONE_HOUR; // Update the time every hour
unsigned long prevNTP = 0;
unsigned long lastNTPResponse = millis();

const unsigned long intervalTemp = 60000;   // Do a temperature measurement every minute
unsigned long prevTemp = 0;
bool tmpRequested = false;
const unsigned long DS_delay = 750;         // Reading the temperature from the DS18x20 can take up to 750ms

uint32_t timeUNIX = 0;                      // The most recent timestamp received from the time server

void loop() {
  unsigned long currentMillis = millis();

  if (currentMillis - prevNTP > intervalNTP) { // Request the time from the time server every hour
    prevNTP = currentMillis;
    sendNTPpacket(timeServerIP);
  }

  uint32_t time = getTime();                   // Check if the time server has responded, if so, get the UNIX time
  if (time) {
    timeUNIX = time;
    Serial.print("NTP response:\t");
    Serial.println(timeUNIX);
    lastNTPResponse = millis();
  } else if ((millis() - lastNTPResponse) > 24UL * ONE_HOUR) {
    Serial.println("More than 24 hours since last NTP response. Rebooting.");
    Serial.flush();
    ESP.reset();
  }

  if (timeUNIX != 0) {
    if (currentMillis - prevTemp > intervalTemp) {  // Every minute, request the temperature
      tempSensors.requestTemperatures(); // Request the temperature from the sensor (it takes some time to read it)
      tmpRequested = true;
      prevTemp = currentMillis;
      Serial.println("Temperature requested");
    }
    if (currentMillis - prevTemp > DS_delay && tmpRequested) { // 750 ms after requesting the temperature
      uint32_t actualTime = timeUNIX + (currentMillis - lastNTPResponse) / 1000;
      // The actual time is the last NTP time plus the time that has elapsed since the last NTP response
      tmpRequested = false;
      float temp = tempSensors.getTempCByIndex(0); // Get the temperature from the sensor
      temp = round(temp * 100.0) / 100.0; // round temperature to 2 digits

      Serial.printf("Appending temperature to file: %lu,", actualTime);
      Serial.println(temp);
      File tempLog = SPIFFS.open("/temp.csv", "a"); // Write the time and the temperature to the csv file
      tempLog.print(actualTime);
      tempLog.print(',');
      tempLog.println(temp);
      tempLog.close();
    }
  } else {                                    // If we didn't receive an NTP response yet, send another request
    sendNTPpacket(timeServerIP);
    delay(500);
  }

  server.handleClient();                      // run the server
  ArduinoOTA.handle();                        // listen for OTA events
}

1. Every hour, the ESP requests the time from an NTP server. Then it constantly checks for a response, and updates the time if it gets an NTP response. If it hasn't received any responses for over 24 hours, there's something wrong, and the ESP resets itself.
2. Every minute, the ESP requests the temperature from the DS18x20 sensor, and sets the 'tmpRequested' flag. The sensor will start the analog to digital conversion.
   750ms after the request, when the conversion should be finished, the ESP reads the temperature from the sensor, and resets the flag (otherwise, it would keep on reading the same temperature over and over again). Then it writes the time and the temperature to a file in SPIFFS.
   By saving it as a CSV file in the filesystem, we can easily download it to the client (using the web server that is running), and it's easy to parse with JavaScript.

Connecting
..........

Connected to SSID
IP address:	192.168.1.2

OTA ready

SPIFFS started. Contents:
	FS File: /favicon-144x144.png, size: 2.81KB
	FS File: /temperatureGraph.js.gz, size: 1.17KB
	FS File: /temp.csv, size: 42.50KB
	FS File: /success.html.gz, size: 456B
	FS File: /edit.html.gz, size: 700B
	FS File: /main.css.gz, size: 349B
	FS File: /index.html.gz, size: 795B
	FS File: /manifest.json, size: 169B
	FS File: /favicon.ico.gz, size: 1.91KB

mDNS responder started: http://esp8266.local
HTTP server started.
Starting UDP
Local port:	123
Time server IP:	216.229.0.179
Sending NTP request
NTP response:	1488666586
Temperature requested
Appending temperature to file: 1488666627,20.00
Temperature requested
Appending temperature to file: 1488666687,19.94
Temperature requested
...