A simple pivot chart doubt

Hi Everybody,

I am using EXCEL 2010 in my system and I am working on some sales data. In view of the large data, I wanted to group my timelines with months instead of dates. So, I went into PIVOT TABLE TOOLS in the menu, selected OPTIONS to use the GROUP FIELD function, but the GROUP FIELD function is disabled in my Excel 2010. I am unable to activate/enable this GROUP FIELD function .

I would really appreciate if folks here can let me know on how to enable/activate the GROUP FIELD function.

Thanks and Cheers.