How Timer Callback Function Does the Timing?

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I want to know how accurate ROS timer is. So for example, I create the timer as follows:

timer1=n.createTimer(ros::Rate(500),&HWController::Timer1_Process,this);

There is a callback function here HWController::Timer1_Process, which takes an argument of C++ class ros::TimerEvent. There are several member variables in this class, one of which is current_expected_. I want to know when current_expected_ is calculated. In ROS documentation, the explanation of this variable is

In a perfect world, this is when the current callback should be happening.

However, no further information is given. I want this accurate value because I need to calculate the time error as shown below.

void HWController::Timer1_Process(const ros::TimerEvent &e)
{
    ros::Duration time_err = e.current_real_ - e.current_expected_;
   .....
}

So, to be more specific, is current_expected_ calculated after this callback function exits (i.e. after the '}' symbol) where it adds the timer period (in this case, 0.0002s) to the current ROS time upon exiting the function? Or is it calculated at the beginning of the function? Actually I prefer to the first explanation because it should not take into account the execution time of this function. I hope someone could clarify this by providing some source codes of its implementation rather than random guessing. I tried to find its implementation but failed. :(