Trying to create clock display

It works for me if you just add a function to pad the extra zero when necessary:

slider = effect("Slider Control")("Slider");
function pad(n){
  return (n < 10 ? "0" : "") + n;
}
x = new Date(1970,0,1); 
x.setSeconds(slider);
x.setMilliseconds(slider % 1 * 100);
pad(x.getHours()) + ":" + pad(x.getMinutes()) + ":" + pad(x.getSeconds()) + ":" + pad(x.getMilliseconds());

if you own iExpression, this expression will be an easy solution:

https://mamoworld.com/after-effects-expression/counter-time

If you want to write your own expression and are looking for ways to add leading zeros to your numbers, see here: https://stackoverflow.com/questions/1127905/how-can-i-format-an-integer-to-a-specific-length-in-javascript