How do I execute some code at a certain time without RTC module?
If you start with the "BlinkWithoutDelay" sketch that comes with the Arduino IDE, you can execute your code alternating between 11 hours and 13 hours.

// Power up the circuit at exactly 06:00.
unsigned long delayInterval = 0;
unsigned long previousMillis = 0;
bool twoHoursAdded = false;

void setup(){

}

void loop(){

  unsigned long currentMillis = millis();

  if(currentMillis - previousMillis &gt;= delayInterval){

    // previousMillis = currentMillis;

    // Use this "method" of incrementing the previousMillis variable
    // instead of the code used in the BlinkWithoutDelay sketch.
    previousMillis += delayInterval; 

    if(twoHoursAdded == false){
      delayInterval = 1000UL * 60UL * 60UL * 13UL;
      twoHoursAdded = true;
    }
    else{
      delayInterval = 1000UL * 60UL * 60UL * 11UL;
      twoHoursAdded = false;
    }

    // Your code here that is run at 06:00 and 19:00.

  }
}


"Down sides" to using this code.


If there is a "power failure", the timing will be off, WAY off until you reboot the Arduino at exactly the right time.
You have to power up the Arduino at exactly the right moment for "it's" timing to match your "clock" timing.


If it's within your budget, have a look at the DS3231 modules available on aliexpress.com / ebay.com. They have a lithium battery backup incase of power outage and are really easy to connect to your Arduino using I2C. There are several libraries on Github that simplify using the DS3231 modules.