Hello Ke Jin,
Thank you for contacting us with your problem. Unfortunatelly, I couldn't reproduce it.
In order to reproduce it i would have to ask you a few questions: Are you using LocalDataSourceProvider, what is this class you are using to hold RowGroupDescriptions, where do you call UpdatePivotGroups(), do you have nullable DateTime data in your source?
I created a sample project using LocalDataSourceProvider as data provider and I am calling UpdatePivotGroups() on a ButtonClick event handler. The problem does not occur.
Can I ask you to reproduce the problem with our application and send it back?