Thread: Low Processor Usage Timing Function

I'm trying to write a function that puts the amount of time in seconds the program has been running into a global variable. I've got one, but it eats CPU time like crazy.

int main()
{
   curTime = 0;
   DWORD nThreadID;
   CreateThread(0, 0, TimeHandler, NULL, 0, &nThreadID);
   return 0;
}

DWORD WINAPI TimeHandler(void* a)
{
   time_t firstTime;
   firstTime = time(NULL);
   for(;;)
   {
      curTime = time(NULL) - firstTime;
   }
}

curTime is a global variable.

There must be a better way to do this. Any suggestions on how to decrease the amount of CPU time this takes, or maybe a totally different approach all together?