Guidance in my project

Hello i have a project and this is the problem ,

now its running for 1 hour and i just wont to change it to run for 30 min can some one advice me thanks .

#include <dht.h>

#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x3F, 16, 2);

const int ledPin = 2;

int ledState = LOW;

unsigned long previousMillis = 0;

const long interval = 3600000;

dht DHT;

#define DHT11_PIN 7

void setup() {

pinMode(ledPin, OUTPUT);

lcd.begin();

}

void loop()

{

unsigned long currentMillis = millis();

if (currentMillis - previousMillis >= interval) {

previousMillis = currentMillis;

if (ledState == LOW)

ledState = HIGH;

else {

ledState = LOW;

}

digitalWrite(ledPin, ledState);

delay(15000);

digitalWrite(ledPin, LOW);

}

int chk = DHT.read11(DHT11_PIN);

lcd.setCursor(0, 0);

lcd.print("Temp: ");

lcd.print(DHT.temperature);

lcd.print((char)223);

lcd.print("C");

lcd.setCursor(0, 1);

lcd.print("Humidity: ");

lcd.print(DHT.humidity);

lcd.print("%");

delay(2000);

I think this

const long interval = 3600000;

is one hour in millisecs. Try changing it

Also it should really be

const unsigned long interval = 3600000;

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...R