google-sheets-appscript-delete-old-rows.js

function prune_old_rows() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getSheetByName('Form Submissions');
  
  var rows = sheet.getDataRange();
  var numRows = rows.getNumRows();
  var values = rows.getValues();
  
  // assumes that there is a column "Added" as the first column
  var addedColumn = 0;

  var currentDate = new Date();
  var twentyOneDaysAgo = new Date();
  twentyOneDaysAgo.setDate(currentDate.getDate() - 21);
  
  var rowsDeleted = 0;
  for (var i = 0; i < numRows; i++){
    var added = values[i][addedColumn];
    if (typeof added.getMonth === 'function' && added  < twentyOneDaysAgo) {
      Logger.log("Deleting row " + i + ".")
      sheet.deleteRow((parseInt(i) + 1) - rowsDeleted);
      rowsDeleted++;
    }
  }
}