I'm new to creating formulas in SalesForce and having trouble with this.  I'd like to start by saying I keep reading about creating formulas through the advanced formula tab, and I don't know where this is which may be my problem.  The only place I know to add a formula is from customizing a report and using the "add formula" option.

That said, in my report I just need to subtract two custom fields, which I believe should be, "Call_Date_Time__c - Date_Assigned__c" but I get an error, "Error: Invalid custom summary formula definition: Field Call_Date_Time__c does not exist."  I found the field name directly from activity custom fields.  Any help would be greatly appreciated.

I'm new to creating formulas in SalesForce and having trouble with this.  I'd like to start by saying I keep reading about creating formulas through the advanced formula tab, and I don't know where this is which may be my problem.  The only place I know to add a formula is from customizing a report and using the "add formula" option.

That said, in my report I just need to subtract two custom fields, which I believe should be, "Call_Date_Time__c - Date_Assigned__c" but I get an error, "Error: Invalid custom summary formula definition: Field Call_Date_Time__c does not exist."  I found the field name directly from activity custom fields.  Any help would be greatly appreciated.