This script is called when I complete tasks on Todoist, and writes Todoist's completed task to google spreadsheet by IFTTT's webhook.

webhookTodoistToGoogleSpreadsheet.gs

// ref: https://internetthingy.com/todoist-ifttt-gas-63.html

function doPost(e) {
  //Todoist->IFTTTからwebhook->GASで受け取るスクリプト
  //データはjson形式で受け取り
  var data = JSON.parse(e.postData.getDataAsString());
  //IFTTTはjsonを3項目までしか出力できないので、タスク名、時間関係データ、それ以外のデータの3項目にした。
  var TaskContent = data.TaskContent;
  var CompletedAt = data.CompletedAt;
  var options = data.options;
  
  CompletedAt = CompletedAt.replace(/January/g,"01")
     .replace(/February/g,"02")
     .replace(/March/g,"03")
     .replace(/April/g,"04")
     .replace(/May/g,"05")
     .replace(/June/g,"06")
     .replace(/July/g,"07")
     .replace(/August/g,"08")
     .replace(/September/g,"09")
     .replace(/October/g,"10")
     .replace(/November/g,"11")
     .replace(/December/g,"12");
  
  //完了日時とスケジュール日時を配列に格納
  var TaskDate = CompletedAt.split( '-' );
  var CompletedDate = convertISO8601(TaskDate[0]);
  var ScheduledDate = convertISO8601(TaskDate[1]);
  
  //その他データを切り出し
  var opt = options.match(/^(.*)-%labels%-(.*)-%priority%-Priority ([1-4]{1})-%url%-(.*)$/);
  var project = opt[1];
  var labels = opt[2];
  var priority = opt[3];
  var url = opt[4];
  
  var taskData = [CompletedDate, TaskContent, ScheduledDate, project, labels, priority, url]
  
  var spread_body = SpreadsheetApp.openById("<ChangeMeToYourSpreadsheetID>").getActiveSheet();
  spread_body.appendRow(taskData); 
}

function convertISO8601(todoistDateFormatString) {
  // 月、日、年、時、分、AM/PMの順に取り出す。
  var dateOpt = todoistDateFormatString.match(/([0-9]{2}) ([0-9]{2}), ([0-9]{4})/);
  var timeOpt = todoistDateFormatString.match(/at ([0-9]{2}):([0-9]{2})([A-Z]{2})/);

  if (timeOpt === null) { // 時間が指定されていないとき
    return Utilities.formatString('%4d-%02d-%02dT00:00:00+09:00', dateOpt[3], dateOpt[1], dateOpt[2]);
  }
  else if (timeOpt[3] === "PM" && timeOpt[1] !== "12") { // 昼の12時は12:00PM
    var tmp = parseInt(timeOpt[1]) + 12;
    timeOpt[1] = tmp.toString(10);
  }
  else if (timeOpt[3] === "AM" && timeOpt[1] === "12") { // 午前0時は12:00AM
    timeOpt[1] = "00";
  }
  return Utilities.formatString('%4d-%02d-%02dT%02d:%02d:00+09:00', dateOpt[3], dateOpt[1], dateOpt[2], timeOpt[1], timeOpt[2]);
}