Calculate the value closest to selected date range

 See if this works for you.

(I've added a Dimension for Warehouse to the model)

Measure for the stock at min date selected:

Stock at Min Selection =
VAR _MinSel =
    MIN ( 'Calendar'[Dates] )
VAR _Stock =
    LASTNONBLANKVALUE (
        FILTER ( ALL ( 'Calendar'[Dates] ), 'Calendar'[Dates] <= _MinSel ),
        [Sum Stock]
    )
RETURN
    _Stock

 Measure for the stock at max date selected:

Stock at Max Selection =
VAR _MaxSel =
    MAX ( 'Calendar'[Dates] )
VAR _Stock =
    LASTNONBLANKVALUE (
        FILTER ( ALL ( 'Calendar'[Dates] ), 'Calendar'[Dates] <= _MaxSel ),
        [Sum Stock]
    )
RETURN
    _Stock

I've attached the sample PBIX file