Please create a Date_Flag calculated field as follows and use this field in the workbook as Date_Flag = 1 always.
Date_Flag = if [New Date] > [Active Date] then 1 else 0 END
Please post .twbx file. This will help in understanding your problem.
I have tried it but still it not working.
Looks like you are blending so Try this:
IF ATTR(New Date)>ATTR(your Second data source.Active Date) Then "True" Else "False" End
Drag it to Label or Filter or Color.
No still it's not working. I have attached Workbook for your reference.
My Goal : If am selecting New Date as Last 1 Month then Active Date should show only the greater date of my "New Date".