SELECT
....
FROM
....
WHERE
(CAST((DATEDIFF(dd, Invoice.DateOpened @EndDate)/30) AS INT) - CAST((DATEDIFF(dd, Invoice.DateOpened, @StartDate)/30) AS INT)) >=1
OR DATEDIFF(dd, Invoice.DateOpened, @StartDate) % 30 = 0 --this line to capture valid records but where From and To dates are the same

SELECT
    ....
FROM
    ....
WHERE
    --In MS T-SQL, BETWEEN is inclusive.
    DateOpened BETWEEN @UserSuppliedFromDate AND @UserSuppliedToDate
    AND DATEDIFF(dd, DateOpened, getdate()) % 30 = 0

DECLARE @Table TABLE
(
    ID integer,
    DateOpened datetime
)

DECLARE @FromDate as datetime = '1/1/2012'
DECLARE @ToDate as datetime = '12/31/2012'

INSERT INTO @Table VALUES (0, '1/1/1982')
INSERT INTO @Table values (1, '1/1/2012')
INSERT INTO @Table VALUES (2, '2/17/2012')
INSERT INTO @Table VALUES (3, '3/16/2012')
INSERT INTO @Table VALUES (4, '4/16/2012')
INSERT INTO @Table VALUES (5, '5/28/2012')
INSERT INTO @Table VALUES (6, '1/31/2012')
INSERT INTO @Table VALUES (7, '12/12/2013')

DECLARE @DateLoop as datetime
DECLARE @ResultIDs as table ( ID integer, DateLoopAtTheTime datetime, DaysDifference integer )

--Initialize to lowest possible value
SELECT @DateLoop = @FromDate

--Loop until we hit the maximum date to check
WHILE @DateLoop <= @ToDate
BEGIN
    INSERT INTO @ResultIDs (ID,DateLoopAtTheTime, DaysDifference)
        SELECT ID, @DateLoop, DATEDIFF(dd,@DateLoop, DateOpened)
        FROM @Table
        WHERE
            DATEDIFF(dd,@DateLoop, DateOpened) % 30 = 0
            AND DATEDIFF(dd,@DateLoop,DateOpened) > 0 -- Avoids false positives when @DateLoop and DateOpened are the same
            AND DateOpened <= @ToDate
    SELECT @DateLoop = DATEADD(dd, 1, @DateLoop) -- Increment the iterator
END

SELECT distinct * From @ResultIDs