Subtracting 2 columns shows incorrect values for blank records - Power Pivot

[montana310]

Hi!

I have a simple a data source that includes product names, sale dates and amounts - pelase find the below data sample with dummy data:

Name	date	Value
test	01.10.2023	1000
test	01.10.2023	10030
test_1	01.11.2023	10004
test_1	01.11.2023	12000
test_2	01.10.2023	3452
test_2	01.11.2023	54000

I have loaded the source data to Data Model in Excel and created a power pivot. As a final step, I want to present the difference between November's and October's values for each product name, therefore I have created DAX messure with the code below:

var current_date = max(Tabela1[date])

var oct_amount=
CALCULATE(

[AMT];

Tabela1[date]=date(2023;10;1)

)

var nov_amount=
CALCULATE(

[AMT];

Tabela1[date]=date(2023;11;1)

)

return

if(current_date=date(2023;10;1);

BLANK();

nov_amount - oct_amount)

The messure shows correct values only for products that have had orders in November and does not read blank values as zero - the incorrect delta value is highlighted in red:

The ISBLANK function also shows a blank result for non-value records:

The data sample is available at the link. 

I would be grateful for any suggestions how to cope with this issue.

[ThxAlot · Accepted solution]

There's an important, profound technology call "Auto-Exist" under the hood. You may want to refer to this article for further understanding.

Understanding DAX Auto-Exist - SQLBI