Increment DateTime Day, Hour, Minute, Second, Millisecond?

I need to increment two datetime's by Day, Hour, Minute, Second, Millisecond.

 Help!

 private void btnLoadListbox_Click(object sender, EventArgs e)
        {
            List<string> list = new List<string>();

            var examDate = new DateTime(2012, 1, 1);
            var expireDate = new DateTime(2015, 1, 1);

            for (int i = 0; i < 100; i++)
            {
                var examDatePlus1Day = examDate.AddDays(1);
                var expireDatePlus1Day = expireDate.AddDays(1);

                list.Add(examDatePlus1Day + "," + expireDatePlus1Day);
            }

            foreach (var s in list)
            {
                listBox1.Items.Add(s);
            }
        }