T = machine cycle * prescaler * postscaler * PR2
Machine cycle = 4 / Fosc

Using a clock of 12Mhz, prescaler 1:16 and postscaler 1:16.
T = (4/12) * 16 * 16 * 255 = 21760us = 21.7ms

void  interrupt () {
PORTB .F1 = ~ PORTB .F1 ; // reverses the state of pin RB1
PIR1 .TMR2IF = 0 ; // clears the interrupt flag
}
	
void  main () {
INTCON .GIE = 1 ; // enable global interruption
INTCON .PEIE = 1 ; // enable interruption of peripherals
PIE1 .TMR2IE = 1 ; // enable interrupt of TIMER2
T2CON = 0b01111111 ; // Postcaler 1:16, TIMER2 on and prescaler 1:16;
TRISB .F1 = 0 ; // define pin RB1 as output
PORTB .F1 = 0 ; / Sets pin RB1 as initial state at low level
PR2 = 255 ; // sets the value for PR2
}

(4/8MHz) * prescaler * (255-96)
0.5 * 32 * 159 = 2544us = 2.544ms
2.544 * 400 times = 1 sec

unsigned cnt;                 	// Define variable cnt

void interrupt() {
    cnt++;                    	// Interrupt causes cnt to be incremented by 1
    TMR0 = 96;               	// Timer TMR0 is returned its initial value
    INTCON = 0x20;           	// Bit T0IE is set, bit T0IF is cleared
}

void main() {

    OPTION_REG = 0x84;         // Prescaler is assigned to timer TMR0
    TRISB = 0;               	// All port B pins are configured as outputs
    PORTB = 0x0;              	// Reset port B

    TMR0 = 96;                	// Timer T0 counts from 96 to 255
    INTCON = 0xA0;            	// Enable interrupt TMR0

    cnt = 0;                  	// Variable cnt is assigned a 0
    
    do {                      	// Endless loop

        if (cnt == 400) {    	// Increment port B after 400 interrupts
            PORTB = PORTB++;  	// Increment number on port B by 1
            cnt = 0;          	// Reset variable cnt
        }

    } while(1);
}

TMR1H = 0x80; // Set initial value for timer TMR1
TMR1L = 0x00;  // TMR1 = 32768
(4/8MHz) * prescaler * (65535 – 32768 ) 
0.5 * 1 * 32767 = 16383.5 us = 16.384 ms
16.384 ms * 76 times = 1.2 sec

unsigned short cnt; // Define variable cnt
void interrupt() {
cnt++ ; // Interrupt causes cnt to be incremented by 1
PIR1.TMR1IF = 0; // Reset bit TMR1IF
TMR1H = 0x80; // TMR1H and TMR1L timer registers are returned
TMR1L = 0x00; // their initial values
}
void main() {
PORTB = 0xF0; // Initial value of port B bits
TRISB = 0; // Port B pins are configured as outputs
T1CON = 1; // Set timer TMR1 , prescaler 1:1 
PIR1.TMR1IF = 0; // Reset bit TMR1IF
TMR1H = 0x80; // Set initial value for timer TMR1
TMR1L = 0x00;
PIE1.TMR1IE = 1; // Enable interrupt on overflow
cnt = 0; // Reset variable cnt
INTCON = 0xC0; // Enable interrupt (bits GIE and PEIE)
do { // Endless loop
if (cnt == 76) { // Change port B state after 76 interrupts
PORTB = ~PORTB; // Number in port B is inverted
cnt = 0; // Reset variable cnt
}
} while (1);
}

(4/Fosc) * prescaler * postscaler * PR2
(4/8MHz) * 16 * 16 * 255 = 32640 us = 32.640 ms 
32.640 * 30 times = 1 sec

unsigned short cnt; // Define variable cnt
void Replace() {
PORTB = ~PORTB; // Define new function ‘Replace’
} // Function inverts port state
void interrupt() {
if (PIR1.TMR2IF) { // If bit TMR2IF = 1,
cnt++ ; // Increment variable cnt by 1
PIR1.TMR2IF = 0;// Reset bit and
TMR2 = 0; // reset register TMR2
}
}
// main
void main() {
cnt = 0; // Reset variable cnt
PORTB = 0b10101010; // Logic state on port B pins
TRISB = 0; // All port B pins are configured as outputs
T2CON = 0xFF; // Set timer2, prescaler=16, postscaler=16,TMR2ON 
TMR2 = 0; // Initial value of timer register TMR2
PIE1.TMR2IE = 1; // Enable interrupt
INTCON = 0xC0; // Set bits GIE and PEIE
while (1) { // Endless loop
if (cnt > 30) { // Change PORTB after more than 30 interrupts
Replace(); // Function Replace inverts the port B state
cnt = 0; // Reset variable cnt
}
}
}