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We will get date in yyyy-mm-dd format 
Indexing starts from zero for month

var date = new Date();
var curr_month  = date.getMonth();
var currentYear = date.getFullYear();

function padTo2Digits(num) {return num.toString().padStart(2, '0');}
function cur_month(date) {
  return [
    date.getFullYear(),
    padTo2Digits(date.getMonth() + 1),
    padTo2Digits(date.getDate()),
  ].join('-');
}

if (curr_month==0)
{var pre_month1 = 11;
var currentYear1 = currentYear-1;}
else {var pre_month1 =curr_month-1 ;
var currentYear1 = currentYear;}

var premonth_start = cur_month(new Date(currentYear1, pre_month1, 1)) );
var premonth_end = cur_month(new Date(currentYear1, pre_month1 + 1, 0))  );