Custom Column to Reverse a Running Total

Please try this expression your column

Change =
VAR __thisvalue = Traps[Count]
VAR __thisdate = Traps[DateCaptured]
VAR __prevdate =
    CALCULATE (
        MAX ( Traps[DateCaptured] ),
        ALLEXCEPT ( Traps, Traps[Trap] ),
        Traps[DateCaptured] < __thisdate
    )
VAR __prevvalue =
    CALCULATE (
        MAX ( Traps[Count] ),
        ALLEXCEPT ( Traps, Traps[Trap] ),
        Traps[DateCaptured] = __prevdate
    )
RETURN
    __thisvalue - __prevvalue

If this works for you, please mark it as the solution.  Kudos are appreciated too.  Please let me know if not.

Regards,

Pat