Why does constant CLOCKS_PER_SEC not need to be namespaced with std:: but

I was trying out the <ctime> library to get a sense of type, clock_t, the clock() function and the constant, CLOCKS_PER_SEC. I noticed that I would have to namespace both clock_t and clock() with std:: but not CLOCKS_PER_SEC. Why is that? How does CLOCKS_PER_SEC get to float on its own?

#include <ctime>
#include <iostream>

int main() {
  std::clock_t start;
  double duration;

  start = std::clock();

  for (long int i = 0; i < 10000000000; i ++){
    // do something
  }

  duration = ( clock() - start ) / (double) CLOCKS_PER_SEC;

  std::cout << duration << std::endl;
}