如何在结束约会日期之前添加提醒？

Question

我需要在结束日期之前添加关于约会的提醒，而不是开始日期。

此约会的创建

private async void AddFavoriteTenderEndDateToCalendar(MyEvent event)
    {
            var appointment = new Appointment();

            appointment.StartTime = new DateTimeOffset(event.StartDate);

            appointment.Duration = event.EndDate - event.StartDate;

            appointment.Subject = "blahblah";

            appointment.Reminder = TimeSpan.FromDays(1);

            var appointmentId = await AppointmentManager.ShowEditNewAppointmentAsync(appointment);           
    }

约会没有属性EndTime，而是我计算持续时间appointment.Duration = event.EndDate - event.StartDate;

如何在event.EndDate之前添加提醒？