Getting error with UDF SQL CLR which returns Datetime result

Question

I have defined two functions

1) Returns String

  [Microsoft.SqlServer.Server.SqlFunction]
public static SqlString TestScalarFunction()
{
    return new SqlString ("RJ");
}

2) Returns Datetime

[Microsoft.SqlServer.Server.SqlFunction]
public static SqlDateTime ScalarUDF()
{
    string zoneId = "Singapore Standard Time";
    TimeZoneInfo tzi = TimeZoneInfo.FindSystemTimeZoneById(zoneId);
    SqlDateTime result = TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, tzi);

    return result;
}

After creating the UDF as::

CREATE FUNCTION [dbo].TestScalarFunction()
RETURNS nvarchar(100)
WITH EXECUTE AS CALLER
AS
EXTERNAL NAME UDF_Blog.UserDefinedFunctions.TestScalarFunction;
GO

CREATE FUNCTION [dbo].ScalarUDF()
RETURNS datetime
WITH EXECUTE AS CALLER
AS
EXTERNAL NAME UDF_Blog.UserDefinedFunctions.ScalarUDF;
GO

I am Getting Problem with only UDF(ScalarUDF) which returns Datetime in its result.

I am using query for both as::

select dbo.TestScalarFunction()  --this successfully gives Result

select dbo.ScalarUDF()   --this gives the Error