Fill values that don't exist with values from another column

I have a table with a column with missing values, I want to fill those missing values with a column in another table.

I created this sample data as an example (sample_data.pbix) : 

Table A

Table B

dates

The relationship between the tables are as follows using "date" as the key between them.

what I need to do is create a column with the values of column "value_A" from Table A, and fill the missing values (in this case the value for September 14th) with the values of "value_B". In this case, the column should have the values: 1623, 9084, 9584, for each date respectively.

I'm not really sure how to determine when a row in Table A doesn't exist. Table dates has the complete set of dates but I should leave that table intact. Table B may or may not have all the values, so I should leave a 0 if both Table A and Table B don't have that particular date.

Does anyone have directions for me to achieve this?

Thanks in advance.