review.js

function doPost(request) {
  var subject;
  var calendarName = 'gas_review';

  if (request) {
    subject = request.parameters.text[0].replace(/done: */, '');
  } else {
    subject = 'test subject';
  }

  var calendar = getCalendar(calendarName);
  var events = [
    calendar.createAllDayEvent(subject + ': 1st review', getDateAfter(1)),
    calendar.createAllDayEvent(subject + ': 2nd review', getDateAfter(7)),
    calendar.createAllDayEvent(subject + ': 3rd review', getDateAfter(30))
  ];

  return ContentService.
    createTextOutput(
      JSON.stringify({
        'text' : 'review events are created',
        'events' : events.map(function(e) { return e.getId(); })
      })
    ).
    setMimeType(ContentService.MimeType.JSON);
}

function getCalendar(name) {
  var calendars = CalendarApp.getAllOwnedCalendars();

  for (var i = 0; i < calendars.length; i++) {
    if (calendars[i].getName() === name) {
      return calendars[i];
    }
  }

  return CalendarApp.createCalendar(name);
}

function getDateAfter(days) {
  var d = new Date();
  d.setDate(d.getDate() + days);
  return d;
}