This uses zeit's ms to convert relation time (eg. 4 days ago) to convert to to preferred time format `10/23/19` using momentjs' luxon

luxon-zeit-ms_time-convert.js

var { DateTime } = require("luxon")
var ms = require("ms")

/**
* this takes a relative time `xxx xxx ago` and returns `xxx xxx`
**/
function stripAgo(date) {
	const trime = date.split(" ");
	// console.log("trime ==> ", trime);
	return trime[2] === "ago" ? `${trime[0]} ${trime[1]}` : date;
}

var n18 = ms(stripAgo("3 days ago")) // send 3 days to `ms`
var dateObj = new Date(); // current date time
var d = dateObj.getTime() - n18 // substract preferred time from current datetime
DateTime.fromMillis(d).toFormat("LL/dd/yy"); // format date to `MM/DD/YY` eg. `10/23/19`

working demo at https://codesandbox.io/s/kind-gould-cxhmf or https://cxhmf.sse.codesandbox.io/