GET DATE FROM STRING WITH FORMAT - JAVASCRIPT or SPREADSHEET

function MAKE_DATE_FROM_STRING(){

  // IF LOGGER WRITE 01/01/1970 -> DATE YOU INPUT DOES NOT EXIST

  var da = new Date(dd_mm_yyyy("31/12/2013"));
  var newda = Utilities.formatDate(da, "GMT+7", "dd/MM/yyyy")
  Logger.log(newda);
  var da = new Date(dd_mm_yyyy("31/02/2013"));
  var newda = Utilities.formatDate(da, "GMT+7", "dd/MM/yyyy")
  Logger.log(newda);
}

function dd_mm_yyyy(ddmmyyyy){
  if(typeof(ddmmyyyy) === "undefined"){return 0}             // CHECK PARAMETER IF NULL WILL RETURN 0
  ddmmyyyy = Trim(ddmmyyyy).toString();                      // TRIM LEFT AND RIGHT DATE INPUT
  if(ddmmyyyy == ""){return 0}                               // IF DATE INPUT IS EMPTY WILL RETURN 0
  ddmmyyyy = ddmmyyyy.match(/^(\d{2})\/(\d{2})\/(\d{4})$/);
  if(ddmmyyyy == null){return 0}                             // IF DATE INPUT STRUCTURE NOT SAME DD/MM/YYYY WILL RETURN 0

  // GET VALUE INTEGER OF DAY, MONTH, YEAR

  var dd = parseInt(ddmmyyyy[1]);
  var mm = parseInt(ddmmyyyy[2]);
  var yyyy = parseInt(ddmmyyyy[3]);

  if(dd < 1 || dd > 31){return 0;}                           // IF DAY < 1 OR DAY > 31 WILL RETURN 0
  if(mm < 1 || mm > 12){return 0;}                           // IF MONTH < 1 OR DAY > 12 WILL RETURN 0
  if(yyyy < 1970){return 0;}                                 // IF YEAR < 1970 WILL RETURN 0

  var kq = new Date(yyyy,(mm-1),dd);                         // MAKE NEW DATE

  // CHECK NEW DATE IS VALID

  if(kq.getDate() != dd){return 0;}
  if(kq.getMonth() != (mm-1)){return 0;}
  if(kq.getFullYear() != yyyy){return 0;}

  return kq.getTime();
}
function Trim(string){
  if(typeof(string) === "undefined"){return ""}
  return string.toString().replace(/^\s\s*/,'').replace(/\s\s*$/,'');
}

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