These are rally two different questions but if you are using a timer in your program both of them can be addressed in the timer callback.
While inside the timer callback, the time from the last execution of the callback is received in eventData2 parameter, while eventData1 parameter holds the timer from the very first execution of the timer: if the timer starts together with the program, this second value approximates the program execution time.
To get the values, cast them into a double variable:
int CVICALLBACK timerCallback (int panel, int control, int event,
void *callbackData, int eventData1, int eventData2)
{
double tscn, ttot;
tscn = *(double *)eventData2; // Time from LAST timer callback
ttot = *(double *)eventData1; // Time from FIRST timer callback
// Other code here
return 0;
}
Depending on the precision you expect, you can implement a countdown by accumulating tscn variable until expected time has elapsed: this will introduce an approximation error that as of my experience is negligible for long intervals (order of magnitude of hours or minutes).
With these limitations, this method is simple and fast and does not implies neither a dedicated timing hardware nor special coding like the use of performance counters and so on.
This method can be ported to asynchronous timers, thus gaining better stability and independency from the user interface events.
