PIC - timer0 and interrupt to count and blink LED - pic

Question

I'm using PIC10F322 and timer0 with interrupt, trying to blink an LED, on for 1s and then off for another 1s. I had tried coded the timer called for interrupt, and it work good. But the counting for time isn't correct. The counting was On for about 2s and then Off for another 2s, which is isn't correct. I wonder is there something wrong with the calculation? The chip is 16MHZ, timer0 is 8 bits and set prescaler to 256. My attempt is to trigger interrupt every 1ms, then do a 999 loop count to hit 1 second.
My calculation is:
256 - [(Delay * Fosc) / (prescaler*4)] = 256 - [(1ms * 16000000)/(256*4)] = 240
#define _XTAL_FREQ 16000000
#include <xc.h>
#pragma config FOSC = INTOSC // Oscillator Selection bits (INTOSC oscillator: CLKIN function disabled)
#pragma config BOREN = OFF // Brown-out Reset Enable (Brown-out Reset disabled)
#pragma config WDTE = OFF // Watchdog Timer Enable (WDT disabled)
#pragma config PWRTE = OFF // Power-up Timer Enable bit (PWRT disabled)
#pragma config MCLRE = OFF // MCLR Pin Function Select bit (MCLR pin function is MCLR)
#pragma config CP = OFF // Code Protection bit (Program memory code protection is disabled)
#pragma config LVP = OFF // Low-Voltage Programming Enable (High-voltage on MCLR/VPP must be used for programming)
#pragma config LPBOR = ON // Brown-out Reset Selection bits (BOR disabled)
#pragma config BORV = LO // Brown-out Reset Voltage Selection (Brown-out Reset Voltage (Vbor), low trip point selected.)
#pragma config WRT = OFF // Flash Memory Self-Write Protection (Write protection off)
int z = 0,count=0;
void main(void) {
ANSELA = 0x00;
TRISA = 0b0100;
TRISAbits.TRISA2 = 1;
LATAbits.LATA0 = 0;
INTCONbits.GIE=1; /* Enable Global Interrupt*/
INTCONbits.PEIE = 1; /* Enable peripheral Interrupt */
OPTION_REGbits.T0CS = 0;
OPTION_REGbits.PSA = 0;
OPTION_REGbits.PS0 = 1; /* set prescaler to 256 */
OPTION_REGbits.PS1 = 1;
OPTION_REGbits.PS2 = 1;
OPTION_REGbits.INTEDG = 0;
TMR0 = 240;
INTCONbits.TMR0IF = 0;
INTCONbits.TMR0IE = 1;
while(1){
}
return;
}
void __interrupt(high_priority) tcInt(void)
{
if (TMR0IE && TMR0IF)
{
TMR0 = 240;
TMR0IF = 0;
if (count == 999)
{
z = 0;
LATAbits.LATA0 = ~LATAbits.LATA0;
count =0;
}
count++;
}
return;
}

Answer 1

You timer calculation is fine but I guess your microcontroller is running at 8 MHz which is the default value after reset if you are working with the internal oscillator (#pragma config FOSC = INTOSC). If you like 16 MHz, you had to do the selection in the OSCCON register.
OSCCON = 0b01110000;