Fixing Group Error on Pivot Table Excel

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Job Description

I have several pivot tables on excel.. I have them group by month..everything works good just one is not's exaclty like the other ones but in that one I get the Can not group that selection error.. data is formated as date and i even tried having no blanks but no luck. write pivot table blue at the beginning of your cover letter.

I need somebody to fix it so it can be group by month.. need this done withing 30 min.