function createCalendarEvent(dateTimeStart, dateTimeEnd, appointment_type) {
  return new Promise((resolve, reject) => {
    calendar.events.list({
      auth: serviceAccountAuth, // List events for time period
      calendarId: calendarId,
      timeMin: dateTimeStart.toISOString(),
      timeMax: dateTimeEnd.toISOString()
    }, (err, calendarResponse) => {
      // Check if there is a event already on the Calendar
      if (err || calendarResponse.data.items.length > 0) {
        reject(err || new Error('Requested time conflicts with another appointment'));
      } else {
        // Create event for the requested time period
        calendar.events.insert({
          auth: serviceAccountAuth,
          calendarId: calendarId,
          resource: {
            summary: appointment_type + ' Appointment', description: appointment_type,
            start: { dateTime: dateTimeStart },
            end: { dateTime: dateTimeEnd }
          }
        }, (err, event) => {
          if (err !== null) {
            reject(err);
          } else {
            resolve(event);
          }
        });
      }
    });
  });
}