RETURN CAST(CAST(@yyyymmdd AS Varchar) AS DateTime)

Complete code of the function (with time parameter):

CREATE FUNCTION [dbo].[F_Convert_int_to_datetime]
(
–parameters for the function
@yyyymmdd int,
@hhmmss int
)
RETURNS DateTime
AS
BEGIN
— Calculate and return the result of the function
IF ISDATE(@yyyymmdd) = 1
BEGIN
RETURN CAST(
CAST(@yyyymmdd as varchar) + ‘ ‘ +
CAST(@hhmmss/10000 as varchar) + ‘:’ +
CAST((@hhmmss/100)%100 as varchar) + ‘:’ +
CAST((@hhmmss%100) as varchar)
as datetime)
END
— ELSE:
RETURN NULL
END

Greets
-Junior Tom