[ARM] ARM 32 bit Timer configuration

[amsbhole07]

I am using LPC1768 IC and external crystal 12 MHZ .
I am using Timer and i want to calculate exact time

timer.c file

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Code C - [expand]
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*****************************************************************************
** Function name:       delayMs
**
** Descriptions:        Start the timer delay in milo seconds
**                      until elapsed
**
** parameters:          timer number, Delay value in milo second             
**                      
** Returned value:      None
** 
*****************************************************************************/
void delayMs(uint8_t timer_num, uint32_t delayInMs)
{
  if ( timer_num == 0 )
  {
    LPC_TIM0->TCR = 0x02;       /* reset timer */
    LPC_TIM0->PR  = 0x00;       /* set prescaler to zero */
    LPC_TIM0->MR0 = delayInMs * (9000000 / 1000-1);
    LPC_TIM0->IR  = 0xff;       /* reset all interrrupts */
    LPC_TIM0->MCR = 0x04;       /* stop timer on match */
    LPC_TIM0->TCR = 0x01;       /* start timer */
  
    /* wait until delay time has elapsed */
    while (LPC_TIM0->TCR & 0x01);
  }
  else if ( timer_num == 1 )
  {
    LPC_TIM1->TCR = 0x02;       /* reset timer */
    LPC_TIM1->PR  = 0x00;       /* set prescaler to zero */
    LPC_TIM1->MR0 = delayInMs * (9000000 / 1000-1);
    LPC_TIM1->IR  = 0xff;       /* reset all interrrupts */
    LPC_TIM1->MCR = 0x04;       /* stop timer on match */
    LPC_TIM1->TCR = 0x01;       /* start timer */
  
    /* wait until delay time has elapsed */
    while (LPC_TIM1->TCR & 0x01);
  }
  return;
}
 
 
/******************************************************************************
** Function name:       Timer0_IRQHandler
**
** Descriptions:        Timer/Counter 0 interrupt handler
**
** parameters:          None
** Returned value:      None
** 
******************************************************************************/
void TIMER0_IRQHandler (void) 
{
 
///////////////////////////////////////
 
 
main file 
 
nt main (void)
{
    SystemInit();
    SystemClockUpdate();
    init_timer ( 0,71999999);
 
        
        enable_timer( 0 );
      
        
        
        LPC_TIM0->TCR = 1;
        NVIC_EnableIRQ(TIMER0_IRQn);
    
  while(1)
  {
 
  }
}

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My query is how to calculate time interval .
what is the clk value
Time interval 71999999=1sec i got from oscilloscope .

please explain me how to calculate exact value