//RTC variables 
#define XTAL_FREQUENCY  12000000 
#define TIMER1_FREQUENCY (XTAL_FREQUENCY / 4)      // 1 clock tick = 1 instr. cycle = crystal frequency / 4 
int32 Ticker; 
int8 Seconds=0; 

//optional: 
int8 Year=0,Month=0,Days=0,Hours=0,Minutes=0; 

//////////////////////////////////////////////////////////////////////////////// 
// Test whether a given year is a leap year. 
// This optimized version only works for the period 2001 - 2099 
//////////////////////////////////////////////////////////////////////////////// 
#define IS_LEAP(year) (year%4 == 0) 


//////////////////////////////////////////////////////////////////////////////// 
//    Initialize RTC 
//////////////////////////////////////////////////////////////////////////////// 
void Initialize_RTC(void) 
{ 
  Ticker = TIMER1_FREQUENCY;                  // initialize clock counter to number of clocks per second 
  setup_timer_1( T1_INTERNAL | T1_DIV_BY_1 ); // initialize 16-bit Timer1 to interrupt 
                                              // exactly every 65536 clock cycles 
                                              // (about 76 times per second) 
  enable_interrupts( INT_TIMER1 );            // Start RTC 
} 

//////////////////////////////////////////////////////////////////////////////// 
//  -=Process Zero Drift Real Time Clock Information=- 
// 
// Most algorithms configure the timer to generate an interrupt every 100ms, and 
// then count the number of interrupts. The problem in that approach is that most 
// clock frequencies can't be divided by 256 and you don't get an exact 100ms. 
// The small errors will add up to an error of several seconds a day. 
// 
// The algorithm presented here is exact in the long run because it doesn't 
// count the number of interrupts but counts the number of clock cycles. 
//////////////////////////////////////////////////////////////////////////////// 
#int_TIMER1                                
void TIMER1_isr()                          
{ 
  Ticker -= 65536;                        // Decrement ticker by clocks per interrupt 
  if ( Ticker < 65536 )                   // If second has expired 
  {  Ticker += TIMER1_FREQUENCY;          //   Increment ticker by clocks per second 
     seconds++;                           //   Increment number of seconds 
  } 

  /* --- Optional part start --- */
  if(Seconds == 60) {Minutes++; Seconds=0; 
    if(Minutes == 60) {Hours++; Minutes=0; 
      if(Hours == 24) {Days++; Hours=0; 
        if (  (Days == 29 && Month==2 && !IS_LEAP(Year)) 
           || (Days == 30 && Month==2) 
           || (Days == 31 && (Month==4 || Month==6 || Month==9 || Month==11 )) 
           || (Days == 32) 
           ) {Month++;Days=0;} 
        if(Month == 13) {Year++; Month=0;} 
  }}} 
   /*--- Optional part end ---  */ 
}