Why date formats miss matching when adding months

When i am getting current date 28-Mar-2015 formated into txtActDate then adding months to it then i am not understanding why its getting in this 28-Mar-15 format.

DateTime dateTime = DateTime.UtcNow.Date;
txtActDate.Text = dateTime.ToString("dd/MMM/yyyy");
DateTime firstDate = DateTime.ParseExact(txtActDate.Text, "dd/MMM/yyyy", null);
firstDate = firstDate.AddMonths(0);
txtAccExp.Text = firstDate.ToShortDateString();