How to make delta time?

note: i am not new to the forums, this is simply my alt for school.
for my car game, which is supposed to run at 60fps, whenever a crappy computer plays the game, it runs more at 30fps, and so all objects go 50% slower. i have a time system using Date.now()

 this.now = 0;
 this.last = 0;
 this.now = Date.now();
 this.last = Date.now();
 project.delay = 0;
 project.fps = 0;

update:

    this.last = this.now;
    this.now = Date.now();
    project.delay = this.now - this.last;
    project.fps = (1000 / project.delay).toFixed(0);
    delaytext.setText("tick delay:" + project.delay + " fps:" + project.fps);

that gives delay in milliseconds and framerate, but i need to use this to make a deltatime function that, when multiplying the speeds of all my objects, counteracts the lag. it would need to be 1 at 60fps, 2 at 30fps, etc. i dont know how to do this, help would be appreciated.
my main account is @monocle_enjoyer btw