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Hi agarwala  If I understood correctly, to provide the flexibility of choosing dates along with the existing logic, you can use a combination of slicers and measures in Power BI.

First create a date slicer with between slicer settings. Then create two measure for start and end date:

SelectedStartDate = 
MIN('DateTable'[Date])

SelectedEndDate = 
MAX('DateTable'[Date])

Here is the image:

Modify your measure to use selected date if select else default calculation:

Waste Quantity (t) =
VAR SelectedMonths = SELECTEDVALUE('Last Months Selection'[Number Of Months (Excludes Current Month)], 18) --- Default to 18

VAR CurrentDate = TODAY()
VAR StartDate = EDATE(CurrentDate, -SelectedMonths)
VAR EndDate = EOMONTH(CurrentDate, -1)

VAR DynamicStartDate = IF(ISBLANK([SelectedStartDate]), StartDate, [SelectedStartDate])
VAR DynamicEndDate = IF(ISBLANK([SelectedEndDate]), EndDate, [SelectedEndDate])

RETURN
CALCULATE(
    SUM(WASTE_API_DATA[Original Waste Quantity (t)]),
    WASTE_API_DATA[ACTIVITY_DATE] >= DynamicStartDate &&
    WASTE_API_DATA[ACTIVITY_DATE] <= DynamicEndDate
)

Hope this helps!!

If this solved your problem, please accept it as a solution and kudos!!

Best Regards,
Shahariar Hafiz