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. In this case, the day part 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. This implementation supports only the current era. Therefore, ArgumentException is thrown 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 October, which has 31 days, the specified day is the thirty-first day of that month, and the value of the months parameter is 6, the resulting year is one more than the specified year, the resulting month is April, and the resulting day is the thirtieth day, which is the last day in April.

If the value of the months parameter 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: 2.0.0.0, 4.0.0.0
Since: .NET 2.0