mega clock?

Using Arduino Project Guidance
1

hello..i want to ask about arduino mega..if i want to create clock on my project should i buy rtc shield or just make some coding?

2

both can work,
one with an RTC is more precise, especially the DS3231 (do not forget the battery)

3

Here's a sketch I use for testing time tracking.
Set the time a little ahead of actual time and download. Open the serial monitor a couple seconds before your pre-set time.

unsigned long currentmicros = 0;
unsigned long nextmicros = 0;
unsigned long interval = 10000; // adjusted for my board
int years = 2013;
int days = 334;
byte tens_hours = 2; // set to Friday 11/29/13, 9:04 pm east coast US
byte ones_hours = 3;  
byte tens_minutes = 0;
byte ones_minutes = 9;
byte tens_seconds = 4;
byte ones_seconds = 0;
byte tenths = 0;
byte hundredths= 0;

byte prior_seconds = 0;

void setup()

{
  Serial.begin(115200);
  nextmicros = micros();
}

void loop()

{

  currentmicros = micros(); // read the time.

  if ((currentmicros - nextmicros) >= interval) // 10 milliseconds have gone by

  {

    hundredths = hundredths +1;

    if (hundredths == 10){
      hundredths = 0;
      tenths = tenths +1;
    }

    if (tenths == 10){
      tenths = 0;
      ones_seconds = ones_seconds +1;
    }

    if (ones_seconds == 10){
      ones_seconds = 0;
      tens_seconds = tens_seconds +1;
    }

    if (tens_seconds == 6){
      tens_seconds = 0;
      ones_minutes = ones_minutes +1;
    }

    if (ones_minutes == 10){
      ones_minutes = 0;
      tens_minutes = tens_minutes +1;
    }

    if (tens_minutes == 6){
      tens_minutes = 0;
      ones_hours = ones_hours +1;
    }

    if (ones_hours == 10){
      ones_hours = 0;
      tens_hours = tens_hours +1;
    }
    if ((tens_hours == 2) && (ones_hours == 4)){
      ones_hours = 0;
      tens_hours = 0;
      days = days +1;
      if (days == 365){
        years = years =1;
      }
    }

    nextmicros = nextmicros + interval; // update for the next comparison

  }  // end time interval check

  // counters are all updated now, send to display

  if (prior_seconds != ones_seconds){
    Serial.print ("Year: ");
    Serial.print (years);
    Serial.print (" Day of the year: ");
    Serial.print (days);
    Serial.print (" Time: ");
    Serial.print (tens_hours, DEC);
    Serial.print (" ");
    Serial.print (ones_hours, DEC);
    Serial.print (" : ");
    Serial.print (tens_minutes, DEC);
    Serial.print (" ");
    Serial.print (ones_minutes, DEC);
    Serial.print (" : ");
    Serial.print (tens_seconds, DEC);
    Serial.print (" ");
    Serial.println (ones_seconds, DEC);

    prior_seconds = ones_seconds;   // show time update once/second
  }  // end one second passing check
  
  // do other stuff in the meantime ...

} // end void loop