Which libraries can count seconds correctly and for which dates?

C++20 can do it with the following syntax:

#include <chrono>
#include <iostream>

int
main()
{
    using namespace std;
    using namespace std::chrono;

    auto t0 = sys_days{2001y/1/1};
    auto t1 = sys_days{2021y/1/1} + 12h + 56min + 23s + 423ms;
    auto u0 = clock_cast<utc_clock>(t0);
    auto u1 = clock_cast<utc_clock>(t1);
    cout << u1 - u0 << '\n';  // with leap seconds
    cout << t1 - t0 << '\n';  // without leap seconds
}

This outputs:

631198588423ms
631198583423ms

The first number includes leap seconds, and is 5s greater than the second number which does not.

This C++20 chrono preview library can do it in C++11/14/17. One just has to #include "date/tz.h", change the y suffix to _y in two places, and add using namespace date;.