Not sure my delta time function in C++ has the right logic

Question

I've been working on making a basic game loop in C++ and I'm unsure my logic is completely right as my delta time isn't what I expect. I expect to get 1/60 seconds per frame as my cap is 60fps, but over the average I take I get a delta time of ~0.03 seconds per frame giving me 33fps. I know the program can run faster than this as if I raise the frame cap it has a smaller delta time, which is still inaccurate. Any help? (I've removed unimportant bits of code to focus on the logic)

using namespace std::chrono;

int main(void)
{   
    //Init time - start and end, with delta time being the time between each run of the loop
    system_clock::time_point startTime = system_clock::now();
    system_clock::time_point endTime = system_clock::now();
    float deltaTime = 0.0f;

    //*Game made here*
  
    /* Loop until the user closes the window */
    while (window is not closed)
    {  
        //Takes time at start of loop
        startTime = system_clock::now();
        deltaFrameCount++;

        //Handle game processes

        //Get time at end of game processes
        endTime = system_clock::now();

        //Take time during work period
        duration<double, std::milli> workTime = endTime - startTime;

        //Check if program took less time to work than the cap
        if (workTime.count() < (milliseconds per frame cap))
        {
            //Works out time to sleep for by casting to double
            duration<double, std::milli> sleepDurationMS((milliseconds per frame cap) - workTime.count());
            //Casts back to chrono type to get sleep time
            auto sleepDuration = duration_cast<milliseconds>(sleepDurationMS);
            //Sleeps this thread for calculated duration
            std::this_thread::sleep_for(milliseconds(sleepDuration.count()));
        }

        //get time at end of all processes - time for one whole cycle
        endTime = system_clock::now();
        duration<double, std::milli> totalTime = endTime - startTime;
        deltaTime = (totalTime / 1000.0f).count();
    }

    //cleans game
    return 0;
}