SQL: Updated IsSmallDate function

Due to a number of unexpected features of the ISNUMERIC function in SQL, I rewrote my IsSmallDate function to not use the function:

--Checks if a string is a valid smalldatetime
ALTER FUNCTION dbo.IsSmallDate
(
  @SmallDateString varchar(20) --The input string to check
)
RETURNS BIT
AS
BEGIN
  DECLARE @Result bit
  SET @SmallDateString = LTRIM(RTRIM(@SmallDateString))
  --Check for numbers only, with a specific pattern
  IF @SmallDateString LIKE '[12][90][0-9][0-9]'
  BEGIN
    IF CAST(@SmallDateString AS int) BETWEEN 1900 AND 2079 
      SET @Result = 1
    ELSE
      SET @Result = 0
    END
  ELSE 
    SET @Result = ISDATE(@SmallDateString)
  RETURN @Result
END