Can you show us the code that you are using to add one month? There are a number of different ways to add durations to a datetime object. You may find the section How Datetime Maths is done from the DateTime documentation will be useful.

Can any one tell me how to get the correct month value

Perhaps that would be easier to do if you told us what you consider the "correct month value" to be. A month isn't a precise duration. It's hard to add one accurately.