Unable to use group the pivot table with date

In addition, validate values, not formats. If you have texts which looks like dates and apply date format to such texts - values will remain texts, applied format changes nothing.

You leave so VERY much unstated here that it's unlikely you'll get much in the way of an explanation. In fact, you don't even ask for an explanation; just offer a vague sentence describing the inability to accomplish a task. Could you help us help you by describing more fully the nature of the grouping you desire. It's clear you expect some kind of Pivot Table grouping around a date value, but not precisely what: group by month, by quarter, by year?  How extensive is the table of data being summarized in the Pivot Table? I've never encountered difficulty in grouping by month, but I'm always dealing with many transactions (usually financial in nature) in each month. What's the nature of your data table in the first place?

Perhaps most helpful to those who might wish to help you: make available a copy of your workbook itself (use OneDrive or GoogleDrive if unable to attach a file here). If the actual file is confidential, then create a meaningful mockup that conceals private/confidential info.