C Sharp Add Days Problem

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C#

In the following code I used AddDays operator to add days. But My requirement is to deduct days from dsStartDate. Can anyone help me, please!!!
if (ddlMaintenPlan.SelectedItem.Text == "Weekly")
{
DateTime dtLastday = Convert.ToDateTime(dtpMaintenceEnd.Text.Trim());


DRLocal = DTLocal.NewRow();
DateTime dtS = Convert.ToDateTime(dtpMaintenceStart.Text.Trim());
DTLocal.Rows.Add(dtS);


while (dsStartDate <= dtLastday)
{
dsStartDate = dsStartDate.AddDays(7);
DRLocal = DTLocal.NewRow();
DTLocal.Rows.Add(dsStartDate);




DTLocal.Rows.Add(dsStartDate);
//}
}



}