what does the function count() do?

[puertas12]

This the program:

#include <iostream>
#include <iomanip>
#include <chrono>
#include <ctime>
#include <thread>
 
// the function f() does some time-consuming work
void f()
{
    volatile double d = 0;
    for(int n=0; n<10000; ++n)
       for(int m=0; m<10000; ++m)
           d += d*n*m;
}
 
int main()
{
    auto t_start = std::chrono::high_resolution_clock::now();
    std::thread t1(f);
    std::thread t2(f); // f() is called on two threads
    t1.join();
    t2.join();
    auto t_end = std::chrono::high_resolution_clock::now();
 
    std::cout << "Wall clock time passed: "
              << std::chrono::duration<double, std::milli>(t_end-t_start).count()
              << " ms\n";
}

I don't understand the count() in this instruction:

std::chrono::duration<double, std::milli>(t_end-t_start).count()

I investigated a bit here:
http://www.cplusplus.com/reference/algorithm/count/
It looks like the function count() compares a range of values with a specific one. This does not make sense for me in this instruction.
Does anybody know what the purpose of count() in the program is?
Thanks

[Peter87]

This is the count function you should be looking at: http://www.cplusplus.com/reference/chrono/duration/count/

The way I understand it is that you create a duration object that use a double to store the time duration in milliseconds, and then you use count() to extract that value.

[puertas12]

Thanks @Peter87