Aggregating a Calculated Field - Based on a Dimension

Hi,

I have created a simple Bucketing calculated field "Less than 30 days" for the following dataset.

PO Number    PO Date            Supplier        PO Amount

1000              01/08/2019          450               100

1001              01/07/2019          450               200

1002              02/08/2019          450               300

1003              02/02/2019          450               400

My calculated field is "Less than 30 days", where i have written a simple formula

= DATEDIFF('day',po date,today())

After which i'm getting the following result.

PO Number    PO Date            Supplier        PO Amount   Less than 30 days

1000              01/08/2019          450               100                    100

1001              01/07/2019          450               200

1002              02/08/2019          450               300                    300

1003              02/02/2019          450               400

But the Column shows '0' when i remove PO Number and PO Date, I would like to see the information at the Supplier level. I understand Tableau calculation works at Row level, Is there a way to address this?

Regards,

SP