Total from Prev Month carrying over into next month

[KW123]

Hi, 

I have this DAX:

VAR _customer = [AccountNbr]

VAR _date = [DisbDate]

VAR _table =

FILTER(

Disb,

Disb[AccountNbr] = _customer

&&Disb[DisbDate] < _date

)

VAR _lastdate =

MAXX( _table, Disb[DisbDate])

VAR _lasttransaction =

MAXX(

FILTER(

_table,

Disb[DisbDate]=_lastdate

),

Disb[Disbamt]

)

RETURN

[DisbAmt] - _lasttransaction

Essentially it is calculating the difference between rows for customer transactions.  

I have run into a small problem.  The last total for the previous month is carrying over into the next month and I need it to not do that.  

For example, in January, on the 31st, the final total is $1000.  On Feb 1st the total should be $10 but it is showing $1010 instead.  The same is happening for March.  

How do I fix the above DAX so that the balance from the last day of the previous month does not carry over to the first day of the next month? I have a separate Dates table too.  But when it will not allow me to replace Disb[DisbDate] with Dates[Date] 

Thank you! 

[Greg_Deckler]

@KW123 Maybe:

Column =
  VAR _customer = [AccountNbr]
  VAR _date = [DisbDate]
  VAR _year = YEAR(_date)
  VAR _month = MONTH(_date)
  VAR _table =
    FILTER(
      Disb,
      Disb[AccountNbr] = _customer && Disb[DisbDate] < _date && YEAR([DisbDate]) = _year && MONTH([DisbDate] = _month
    )
VAR _lastdate =
MAXX( _table, Disb[DisbDate])
VAR _lasttransaction =
MAXX(
FILTER(
_table,
Disb[DisbDate]=_lastdate
),
Disb[Disbamt]
)
RETURN
[DisbAmt] - _lasttransaction

[KW123]

@Greg_Deckler 

Thank you so much for your reply!! 
This is what I get if I copy and paste... 

[Greg_Deckler · Accepted solution]

@KW123 Missed a )

Column =
  VAR _customer = [AccountNbr]
  VAR _date = [DisbDate]
  VAR _year = YEAR(_date)
  VAR _month = MONTH(_date)
  VAR _table =
    FILTER(
      Disb,
      Disb[AccountNbr] = _customer && Disb[DisbDate] < _date && YEAR([DisbDate]) = _year && MONTH([DisbDate]) = _month
    )
VAR _lastdate =
MAXX( _table, Disb[DisbDate])
VAR _lasttransaction =
MAXX(
FILTER(
_table,
Disb[DisbDate]=_lastdate
),
Disb[Disbamt]
)
RETURN
[DisbAmt] - _lasttransaction

[KW123]

@Greg_Deckler 

This is it!! Thank you so so so much