Debugging: function date_part(unknown, integer) does not exist

When creating a formula in Pivot to get number of days between two date fields, using the DDIFF() function, I get the following message when the widget errors out.

function date_part(unknown, integer) does not exist

Note, these are date only fields and not timestamp/datetime.

I filtered out Null values using the widget filter and even tried filtering out in the Formula date fields.

Live connection to a PostgreSQL database.

Update: The formula works if one of the 2 days is a timestamp field. 2 date only fields error out. This seems to be a bug in the function.