月1日目のタイムスタンプを取得します.

//     
function formatDateTime(inputTime) {
  let date = new Date(inputTime);
  let y = date.getFullYear();
  let m = date.getMonth() + 1;
  m = m < 10 ? ('0' + m) : m;
  let d = date.getDate();
  d = d < 10 ? ('0' + d) : d;
  let h = date.getHours();
  h = h < 10 ? ('0' + h) : h;
  let minute = date.getMinutes();
  let second = date.getSeconds();
  minute = minute < 10 ? ('0' + minute) : minute;
  second = second < 10 ? ('0' + second) : second;
  return y + '-' + m + '-' + d + ' ' + h + ':' + minute + ':' + second;
};

//                   
function getMounthData(mounth) {
  let data = new Date();
  data.setDate(1);
  data.setHours(0);
  data.setSeconds(0);
  data.setMinutes(0);
  data.setMonth(mounth - 1)
  console.log(formatDateTime(data.getTime()), '     ：', data.getTime());
}

getMounthData(8)