How to filter a table based on a particular date in power BI

I have the below table in power BI and I want to perform 2 things:

1. I want to filter the table based on date. I.e, I want to filter only 01-Jan values.
2. Once I get the 1st Jan values I want to calculate the difference between 2 dates.

Below is what I have:

. .

I want to achieve below:

---

Below is the table of values I have got after adding a column.

Now I want to calculate the difference between the NAV column rows, with the current measure it is working like below:

Expected Output is :

Issue is that it is not calculating difference for 2021 year.I would like to show negative values as well if applicable, as I'm planning put a data bar for this value.Below is the image link where the table image is clear.Kindly help to get the negative values as well.

[Current working image][1] [1]: https://i.sstatic.net/fzC5W4h6.png

pls try this

measure =
VAR _next =
    MINX (
        FILTER (
            ALL ( 'Table' ),
            'Table'[Date] > MAX ( 'Table'[Date] )
                && 'Table'[Column] = "y"
        ),
        'Table'[Date]
    )
VAR next_value =
    SUMX ( FILTER ( ALL ( 'Table' ), 'Table'[Date] = _next ), 'Table'[NAV] )
VAR _last =
    MAXX ( FILTER ( ALL ( 'Table' ), 'Table'[Column] = "y" ), 'Table'[Date] )
RETURN
    IF (
        MAX ( 'Table'[Date] ) = _last,
        BLANK (),
        next_value - SUM ( 'Table'[NAV] )
    )