Depending on your regional settings, the parameter you are passing in for @TimeItemAdded might not be recognized.

You should pass the date in as:

20120720

Got an answer from elsewhere that I will use:

SELECT top 1 multi FROM OPENQUERY([production], 'DECLARE @dt datetime SELECT @dt = GETDATE() exec proddb.dbo.spCalcProjection @dt')

This avoids having to create any additional objects in the db.

When using OleDb one should always remember that the parameters are not passed according to their names but in the exact order in which they are added to the Parameters collection.

In your code you add first the @repRIS and this is the first parameter passed to the SP. But the SP expects a date for the first parameter and you get the exception

You need to change the order of the insertion in the Parameters collection or switch the declaration order of the parameters in the SP

command.Parameters.Add("@invDt", OleDbType.Date).Value = invDate;     
if (repRIS.Length > 0)
    command.Parameters.AddWithValue("@repRIS", repRIS);
else
    command.Parameters.AddWithValue("@repRIS", DBNull.Value);

And another thing to do is to look at this article Can we stop using AddWithValue already?

You can't pass function as stored procedure parameter. Use variable to store the function return-value, then you can pass the variable as parameter for SP, for example :

DECLARE @currentdate DATETIME
SET @currentdate = GETDATE()

EXEC PP_Sp_ObservationSchedule'F5B42B97-4828-4D0B-8129-0FAA37800ADB',1,1,'F5B42B97-4828-4D0B-8129-0FAA37800ADB',1,@currentdate,'abc',''
                                                                                       Notice usage of the variable^

'28/1/2013' is an ambiguous format - SQL Server might interpret it as dd/mm/yyyy or as mm/dd/yyyy. In this case, it's done the latter, and doesn't know what the 28th month of the year is.

Use '20130128' instead. This is always interpreted as yyyymmdd.

Try CONVERT(DATETIME, @PaymentDate, 103)

104 is the German style which uses periods between the numerals, as opposed to slashes. 103 is the British/French style.

See: https://msdn.microsoft.com/en-us/library/ms187928.aspx

You should be using an open-ended date range instead of converting both sides of the equation to a regional string.

ALTER PROCEDURE [dbo].[myProc]
  @id   INT,
  @date DATETIME
AS
BEGIN
  SET NOCOUNT ON;

  SELECT id, DATEPART(DAY, @date)
    FROM dbo.myTable
    WHERE id = @id
    AND [date] >= DATEADD(DAY, 0, DATEDIFF(DAY, 0, @date))
    AND [date] <  DATEADD(DAY, 1, DATEDIFF(DAY, 0, @date));
END
GO

Now, you can't just pass getdate() into a stored procedure call like that (the syntax is invalid), and you certainly can't pass it as a string. So a couple of options.

(a) Declare a variable up front, assign the current date/time to the variable, and pass the variable in:

DECLARE @d DATETIME;
SET @d = GETDATE();
EXEC [dbo].[myProc] @id = 1, @date = @d;

(b) Make the parameter optional, and then don't bother passing that param in:

ALTER PROCEDURE [dbo].[myProc]
  @id   INT,
  @date DATETIME = NULL
AS
BEGIN
  SET NOCOUNT ON;

  SET @date = DATEADD(DAY, 0, DATEDIFF(DAY, 0, 
    COALESCE(@date, GETDATE())));

  SELECT id, DATEPART(DAY, @date)
    FROM dbo.myTable
    WHERE id = @id
    AND [date] >= @date
    AND [date] <  DATEADD(DAY, 1, @date);
END
GO

Now your call can just be:

EXEC dbo.myProc @id = 1;

Also, you shouldn't use 'single quotes' for column aliases - this syntax is deprecated. Use [square brackets] instead.