I've written this code for a reaction timer game. The reaction time is to be displayed in milliseconds to the serial monitor as accurately as possible. The reaction time is not being displayed correctly. I'm including my timer setup routine and where I increment milliseconds. 

```
TCCR1A=0; 
TCCR1B=0; 
timer1_counter=65536; 
TCNT1=timer1_counter; 
TCCR1B |=(1<<CS12); 

TIMSK1 |=(1<<TOIE1); 
interrupts();  
}

ISR(TIMER1_OVF_vect) 
{
  TCNT1=timer1_counter; 
  int_flag=1;
 }

  digitalWrite(LED,LOW); 
  randomSeed(analogRead(A1)); 
  NumSecs=random(1000,7000);  
  delay(NumSecs);  
  digitalWrite(LED,HIGH);
  while(digitalRead(PUSHBUTTON) == HIGH)  
  {
  }
  if (int_flag==1){
  millisecs++;
  int_flag=0;
  }

```