gistfile1.cs

    public class SpecialDate
    {        
        public DateTime GetThreeMonthLater(DateTime original)
        {
            //msdn reference: http://msdn.microsoft.com/zh-tw/library/system.datetime.addmonths.aspx
            //AddMonths 方法會將閏年和月份天數列入考量以計算結果的月份和日期，然後調整產生之 DateTime 物件的日期部分。
            //如果產生的日期在產生的月份中不是有效的日期，則會使用產生之月份的最後一個有效日期。
            //例如 3 月 31 日 + 1 個月 = 4 月 30 日。產生之 DateTime 物件的當天時間部分仍然與這個執行個體相同。
            var result = original.AddMonths(3);
            return result;
        }
    }

gistfile2.cs

    [TestClass]
    public class UnitTest1
    {
        [TestMethod]
        public void Test2013_03_31()
        {
            var target = new SpecialDateLibrary.SpecialDate();
            var result = target.GetThreeMonthLater(new DateTime(2013, 3, 31));
            var expected = new DateTime(2013, 6, 30);
            Assert.AreEqual(expected, result);
        }

        [TestMethod]
        public void Test2013_04_01()
        {
            var target = new SpecialDateLibrary.SpecialDate();
            var result = target.GetThreeMonthLater(new DateTime(2013,4,1));
            var expected = new DateTime(2013,7,1);
            Assert.AreEqual(expected, result);
        }

        [TestMethod]
        public void Test2013_11_29()
        {
            var target = new SpecialDateLibrary.SpecialDate();
            var result = target.GetThreeMonthLater(new DateTime(2013, 11, 29));
            var expected = new DateTime(2014, 2, 28);
            Assert.AreEqual(expected, result);
        }

        [TestMethod]
        public void Test2013_11_30()
        {
            var target = new SpecialDateLibrary.SpecialDate();
            var result = target.GetThreeMonthLater(new DateTime(2013, 11, 30));
            var expected = new DateTime(2014, 2, 28);
            Assert.AreEqual(expected, result);
        }


        [TestMethod]
        public void Test2015_11_29()
        {
            var target = new SpecialDateLibrary.SpecialDate();
            var result = target.GetThreeMonthLater(new DateTime(2015, 11, 29));
            var expected = new DateTime(2016, 2, 29);
            Assert.AreEqual(expected, result);
        }

        [TestMethod]
        public void Test2015_11_30()
        {
            var target = new SpecialDateLibrary.SpecialDate();
            var result = target.GetThreeMonthLater(new DateTime(2015, 11, 30));
            var expected = new DateTime(2016, 2, 29);
            Assert.AreEqual(expected, result);
        }
    }