This sketch blinks an LED on D10 (on a Uno) at one second intervals. It does not use delay, micros, millis or any unsigned long variables (apart from a constant).

const int STROBE_FREQ = 1000;     // sets the period in milliseconds 
const unsigned long countTo = ((float) F_CPU / 1024.0) / (1000.0 / STROBE_FREQ); 

void setup ()
  {
  // D10 to output
  bitSet (DDRB, 2);

  // Fast PWM top at OCR1A
  TCCR1A = bit (WGM10) | bit (WGM11); // fast PWM
  TCCR1B = bit (WGM12) | bit (WGM13) | bit (CS12) | bit (CS10);   // fast PWM, prescaler of 1024
  OCR1A = countTo - 1;                 // zero relative 
  OCR1B = (countTo / 2) - 1;           // 50% duty cycle
  bitSet (TCCR1A, COM1B1);   // clear OC1B on compare
  }  // end of setup

 void loop ()
   {
    // do something useful
   }

It frees up the main loop to do something useful without worrying about time taken to do the blinking.

This sketch blinks an LED on D10 (on a Uno) at one second intervals. It does not use delay, micros, millis or any unsigned long variables (apart from a constant).

const int STROBE_FREQ = 1000;     // sets the period in milliseconds 
const unsigned long countTo = ((float) F_CPU / 1024.0) / (1000.0 / STROBE_FREQ); 

void setup ()
  {
  // D10 to output
  bitSet (DDRB, 2);

  // Fast PWM top at OCR1A
  TCCR1A = bit (WGM10) | bit (WGM11); // fast PWM
  TCCR1B = bit (WGM12) | bit (WGM13) | bit (CS12) | bit (CS10);   // fast PWM, prescaler of 1024
  OCR1A = countTo - 1;                 // zero relative 
  OCR1B = (countTo / 2) - 1;           // 50% duty cycle
  bitSet (TCCR1A, COM1B1);   // clear OC1B on compare
  }  // end of setup

 void loop ()
   {
    // do something useful
   }

It frees up the main loop to do something useful without worrying about time taken to do the blinking.

For more details see my page about timers.