Absolute time using monotonic clock

For some 3rd party library functions, it requires that I pass it a timespec that uses absolute time and not relative time.

To do this I have:

struct timespec ts;
struct timespec now;
if (clock_gettime(CLOCK_REALTIME, &now) != 0)
{
    printf("Error: ");
    printf("%s\n", strerror(errno));
}
tspec_add(&ts, &now, 5000);  //ts = now + 5000ms.

//I pass ts to the library function.

And the functions work fine.. However, if I use CLOCK_MONOTONIC or CLOCK_MONOTONIC_RAW it does NOT work.

I checked for errors (NONE occur). The function success but the time it outputs is completely different and very very small. It printed 9005 for tv.sec but when using CLOCK_REALTIME it printed a huge number (I assume time since epoch).

Why is this? How can I use the monotonic clock to do the same?