Arduino - Basic task scheduler

unsigned long taskLastRun = 0;
unsigned long taskRunEvery = 20000; // milliseconds

void setup(void)
{
    //
}

void task(void)
{
    // something to do...
}

void loop(void)
{
    if (millis() - taskLastRun >= taskRunEvery)
    {
        task();
        taskLastRun = millis();
    }
}