The day part of the resulting DateTime is affected if the resulting day is not a valid day in the resulting month of the resulting year. It is changed to the last valid day in the resulting month of the resulting year. The year part of the resulting DateTime is affected if the resulting month is outside the year of the specified DateTime. The era part of the resulting DateTime is affected if the resulting year is outside the era of the specified DateTime. The time-of-day part of the resulting DateTime remains the same as the specified DateTime.

For example, if the specified month is Juugatsu (October), which has 31 days, the specified day is the 31st day of that month, and months is 6, the resulting year is one more than the specified year, the resulting month is Shigatsu (April), and the resulting day is the 30th day, which is the last day in Shigatsu (April).

If months is negative, the resulting DateTime is earlier than the specified DateTime.

The DateTime.Kind property of the returned DateTime value always equals DateTimeKind.Unspecified. You can preserve the DateTime.Kind property of the time parameter by calling the DateTime.SpecifyKind(DateTime, DateTimeKind) method, as the following example shows.

code reference: System.Globalization.Calendar.AddMethods#5

Namespace: System.Globalization
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0