ofTimer lets you run code periodically with the specified frequency. Use it on a class that extends ofThread.

class Scheduler: public ofThread {
public:
    Scheduler() {
        timer.setPeriodicEvent(1000000000); // this is 1 second in nanoseconds
        startThread();
    }

private:
    ofTimer timer;
    void threadedFunction() {
        while(isThreadRunning()) {
             timer.waitNext();
             // Do your thing here. It will run once per second.
        }
    }
}

The timer above will wake up at 1 second intervals no matter how long the rest of the task takes. If the last execution took more than 1 second, it'll wake up right away and execute the next. It uses the most accurate timing functions in every OS.