Round to closest multiple of fraction (worded terribly)

Question

Say I have this code for a simple timer:

var a = document.getElementById("a")
var start = Date.now()
window.onload = function() {
  setInterval(()=>{
    a.innerHTML = ((Date.now() - start)/1000).toFixed(3)
  }, 1)
}

<p id="a">0.000</p>       

I've always wanted a timer that counts in multiples of 1/30, so therefore it would go .033, .067, .100, .167, etc. I cannot modify the setInterval just to do this because of how inaccurate it is, so how can I round what the current time is so that the decimal ends in a valid value?
I know this is terribly worded, I apologize