Dax

hello im not sure if i understood you correctly but please try:

Days = 
VAR _min = MIN(Sheet1[Start Date])
VAR _max = MAX(Sheet1[End Date])
Return
COUNTROWS(CALENDAR(_min,_max))-1

make sure to go to format ---> edit interactions and select none on each slicer

now to find post and pre:

Pre Scheme = MIN(Sheet1[Start Date]) - [Days]

Post scheme = MAX(Sheet1[End Date]) + [Days]

and another example:

then to find your desired sales:

Measure = 
CALCULATE (
    SUM ( salestable[sales] ),
    FILTER (
        salestable,
        salestable[Date] >= pre scheme )
            && salestable[Date] <= psotscheme )
    )
)

dont forget to create a relationship between scheme table and salestable

if you find it helpful a kudos would be appreciated and consider accepting as solution