Temp2Firebase.ino

#include <ESP8266WiFi.h>
#include <FirebaseArduino.h>
#include <DHT.h>
#include <time.h>


// Config Firebase
#define FIREBASE_HOST "esp-data-logger.firebaseio.com"
#define FIREBASE_AUTH "<YOUR KEY>"

// Config connect WiFi
#define WIFI_SSID "<YOUR WIFI NAME>"
#define WIFI_PASSWORD "<YOUR WIFI PASSWORD>"

// Config DHT
#define DHTPIN 4
#define DHTTYPE DHT22

// Config time
int timezone = 7;

char ntp_server1[20] = "ntp.ku.ac.th";
char ntp_server2[20] = "fw.eng.ku.ac.th";
char ntp_server3[20] = "time.uni.net.th";

int dst = 0;

DHT dht(DHTPIN, DHTTYPE);

void setup() {
  Serial.begin(9600);

  WiFi.mode(WIFI_STA);
  // connect to wifi.
  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
  Serial.print("connecting");
  
  while (WiFi.status() != WL_CONNECTED) {
    Serial.print(".");
    delay(500);
  }
  Serial.println();
  Serial.print("connected: ");
  Serial.println(WiFi.localIP());

  configTime(timezone * 3600, dst, ntp_server1, ntp_server2, ntp_server3);
  Serial.println("Waiting for time");
  while (!time(nullptr)) {
    Serial.print(".");
    delay(500);
  }
  Serial.println();
  Serial.println("Now: " + NowString());
  
  Firebase.begin(FIREBASE_HOST, FIREBASE_AUTH);
  dht.begin();
}

void loop() {
  // Read temp & Humidity for DHT22
  float h = dht.readHumidity();
  float t = dht.readTemperature();

  if (isnan(h) || isnan(t)) {
    Serial.println("Failed to read from DHT sensor!");
    delay(500);
    return;
  }
  Serial.print("Humidity: ");
  Serial.print(h);
  Serial.print(" %\t");
  Serial.print("Temperature: ");
  Serial.print(t);
  Serial.print(" *C ");
  Serial.println();

  StaticJsonBuffer<200> jsonBuffer;
  JsonObject& root = jsonBuffer.createObject();
  root["temperature"] = t;
  root["humidity"] = h;
  root["time"] = NowString();
  
  // append a new value to /logDHT
  String name = Firebase.push("logDHT", root);
  // handle error
  if (Firebase.failed()) {
      Serial.print("pushing /logs failed:");
      Serial.println(Firebase.error());  
      return;
  }
  Serial.print("pushed: /logDHT/");
  Serial.println(name);
  delay(30000);
}

String NowString() {
  time_t now = time(nullptr);
  struct tm* newtime = localtime(&now);

  String tmpNow = "";
  tmpNow += String(newtime->tm_year + 1900);
  tmpNow += "-";
  tmpNow += String(newtime->tm_mon + 1);
  tmpNow += "-";
  tmpNow += String(newtime->tm_mday);
  tmpNow += " ";
  tmpNow += String(newtime->tm_hour);
  tmpNow += ":";
  tmpNow += String(newtime->tm_min);
  tmpNow += ":";
  tmpNow += String(newtime->tm_sec);
  return tmpNow;
}

Why have error at

• Serial.println("Now: " + NowString());
• root["time"] = NowString();