在 C# 中获取两个特定日期时间的截止日期前 3 周

我有两个日期作为开始日期和结束日期。我需要在我设置的结束日期/截止日期前 3 周发送提醒。

var start= DateTime.Now;
var end = new DateTime(2016, 10, 25);
var timeParts = StartEndDate.Split(new char[1] { ':' });
var eventstartDate = new DateTime(start.Year, start.Month, start.Day,
int.Parse(timeParts[0]), int.Parse(timeParts[1]), int.Parse(timeParts[2]));

var eventEnDate = new DateTime(end .Year, end .Month, end .Day,
int.Parse(timeParts[0]), int.Parse(timeParts[1]), int.Parse(timeParts[2]));
TimeSpan ts = eventEnDate - eventstartDate;
int totaldays = ts.Days;
int totalWeeks = totaldays / 7;
//Add 7 days for each loop until end date
for (DateTime ds= start; ds< eventEnDate; ds= ds.AddDays(7))
{
    //DO something so to get last 3 week prior to end date to set reminder
}

var endDateMinusThreeWeeks = end.AddDays(-21);