Subtract two dates from different tables indirectly related

[C-Jac]

I have 4 tables: 

Invoice Calendar

Invoices 

Projects

Complete Calendar

They are each related, the Invoice Calendar is related to Invoices throug a calendar ID, the Invoices are related to each Project using a Project ID, and each Project is related to the Complete Calendar through a Calendar ID. 

What I need is to make a measure which will show me the difference between the Complete Date and Invoice Date in the two tables, and I need to show it like this in Power BI: 

Project	Invoice	Complete Date	Invoice Date	Diff
1	1	01/01/2021	02/01/2021	1
1	2	01/01/2021	03/01/2021	2
1	3	01/01/2021	06/01/2021	5
2	1	05/01/2021	08/01/2021	3

I've tried setting variables etc. but I cant make it select the column. And I want the connection to be live, and I don't have access to the data model, so it has to be a measure I can put in. 

[stevedep]

Lookup value?

https://dax.guide/lookupvalue/ 

[Anonymous]

[Diff] =
var OneProjectVisible = HASONEVALUE( Projects[Project ID])
var OneInvoiceVisible = HASONEVALUE( Invoices[Invoice ID] )
var Result =
    if( OneProjectVisible && OneInvoiceVisible,
        var CompleteDate =
            CALCULATE(
                SELECTEDVALUE( 'Complete Calendar'[Date] ),
                Projects
            )
        var InvoiceDate =
            CALCULATE(
                SELECTEDVALUE( 'Invoice Calendar'[Date] ),
                Invoices
            )
        return
            // Assumption is that all projects and all
            // invoices do have non-blank invoice dates
            // and completion dates. If not, you'll have
            // to adjust this formula to account for cases
            // where this assumption is not true. And, of
            // course, I assume that an invoice can have
            // at most 1 invoice date. Same is true for
            // projects - there's at most only one completion
            // date for each of them.
            InvoiceDate - CompleteDate
    )
return
    Result

[amitchandak]

@C-Jac , you need to do it using a common table/s

Refer this: https://community.powerbi.com/t5/Community-Blog/Decoding-Direct-Query-in-Power-BI-Part-2-Date-Differ...

same is true for import mode