c++ - How to calculate delta time with SDL?

I have heard that you can calculate delta time with two functions called SDL_GetPerformanceCounter and SDL_GetPerformanceFrequency. I'm not sure how they work completely, but I heard it is a bit similar to using SDL_GetTicks()

    Uint64 NOW, LAST;
    double deltaTime = 0;

    while (somebool)
    {
       LAST = SDL_GetPerformanceCounter();
       NOW = SDL_GetPerformanceCounter();
       LAST = NOW;


       deltaTime = (NOW - LAST) / (double)SDL_GetPerformanceFrequency();

       /**Rendering**/
    }

I have this code but whenever I print out deltaTime or try to use it it always equals some really random and insane numbers. Is this correct?