arm a timer with initial expiration set to 0, doesnt work

[jenia.ivlev]

Hello.

I'm going through the book Posix Programming interface.
When I set the timer timer_t, I set the itimerspec's first expiration to
1, and run the program, the timer works fine. When I set it to 0, it
doesnt work anymore.

Let me show you what I mean:

    void itimerspecFromStr(char *interval, struct itimerspec *tsp)
    {
        tsp->it_value.tv_sec = 1; //<-------------- when its set to one, it works.
        tsp->it_value.tv_nsec = 0;
        tsp->it_interval.tv_sec = atoi(interval);
        tsp->it_interval.tv_nsec = 0;
    }

When I set that value to zero, and run the same program, it doesnt work
anymore.
Is that normal?

Thanks in advance.