cc2640 timer

In sensortag application code ,we can see the code below .I want to know how to guarantee 

keyRight.tStop greater than keyRight.tstart .And I want to know how to use timer interrupt.

when does the function

SensorTag_callback  execute??

(keyRight.tStop - keyRight.tStart)

void SensorTagKeys_processKeyLeft(void)
{
  if (PIN_getInputValue(Board_KEY_LEFT))
  {
    keys &= ~SK_KEY_LEFT;
    keyLeft.tStop = Clock_getTicks();
  }
  else
  {
    keys |= SK_KEY_LEFT;
    keyLeft.tStart = Clock_getTicks();
  }
  
  // Wake up the application thread
  Semaphore_post(sem);
}

 duration = ((keyRight.tStop - keyRight.tStart) * Clock_tickPeriod) 
          / 1000;



static void SensorTag_callback(PIN_Handle handle, PIN_Id pinId)
{
  switch (pinId) {

  case Board_KEY_LEFT:
    SensorTagKeys_processKeyLeft();
    break;

  case Board_KEY_RIGHT:
    SensorTagKeys_processKeyRight();
    break;

  case Board_RELAY:
    SensorTagKeys_processRelay();
    break;

  default:
    /* Do nothing */
    break;
  }
}