Round to the nearest time interval using moment.js

roundDown.js

function roundDown (ts, duration) {
	const now = moment(ts);
  const dur = duration.split (" ");
  dur [0] = +dur[0];

	const rest = (now[dur[1]]() % dur[0])
  const start = now.clone().subtract(rest, dur[1]).startOf(dur[1])

  return +start;
}

roundDown(+new Date, '15 minutes')