Non Deterministic Function?

Gentlemen What is "Non_Deterministic" about the function below?

I pass DATETIME Column and a DECIMAL column ti the function. It keeps yelling at me saying it is a non-deterministic function.

I am using this function to PERSIST a Computed Column.

I have tried converting all NVARCHARs to VARCHARs.

Tredi returning a VARCHAR instead of a DATETIME, but still did not succeed.

Am I doing something wrong, I must be.....

CREATE FUNCTION [dbo].[udf_GetDateTime](@Date DATETIME, @TimeDecimal DECIMAL)

RETURNS DATETIME

AS

BEGIN

DECLARE @DateStr NVARCHAR(23)

DECLARE @TimeStr NVARCHAR(12)

DECLARE @DateTimeResult DATETIME



SET @TimeStr = RIGHT('000000' + CONVERT(NVARCHAR(6), @TimeDecimal), 6)

SET @DateStr = CONVERT(NVARCHAR(10), @Date, 120) + ' ' +

SUBSTRING(@TimeStr, 1, 2) + ':' +

SUBSTRING(@TimeStr, 3, 2) + ':' +

SUBSTRING(@TimeStr, 5, 2)



RETURN CONVERT(DATETIME, @DateStr, 120)

END