It does not even allow an insert into a temp table. The clr returns a single value, thus:

CREATE TABLE #Temp (

tValue NUMERIC(18,4))

INSERT INTO #Temp

SELECT fncFormula(5, 2014, 6, 28)

or

SELECT fncFormula(5, 2014, 6, 28)

INTO #Temp

Both fail because it inserts a zero value, but "SELECT fncFormula(5, 2014, 6, 28)" returns the correct value.