Conversion failed when converting datetime from character string when calling SQL function in ASP.net

I am trying to get my first SQL function working from ASP.net . In the ASP.net application the user clicks the calendar control and I want to fill a dataset with the SQL function using the date selected. I keep getting the error described in the title when the dataset is to be filled. With a msgbox I can see the date being passed to the funcition in the format mm\dd\yyyy. Please help.
 

Protected Sub Calendar1_SelectionChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles Calendar1.SelectionChanged
        Dim StartDate As Date
        StartDate = Calendar1.SelectedDate.Date

        Dim cnstr2 As String = "Data Source=ELECTRICAL-PRO;Initial Catalog=Power;Integrated Security=True"
        Dim sqlcon As New SqlClient.SqlConnection(cnstr2)
        sqlcon.Open()
        Dim str As String = "SELECT * FROM udfDailyKwHrUsage('@" & StartDate & "')"
        Dim cmd As New SqlClient.SqlCommand(str, sqlcon)
        Dim ds As New DataSet()
        Dim da As New SqlClient.SqlDataAdapter(cmd)
        da.Fill(ds) 'Conversion failed when converting datetime from character string
        
End Sub



set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
go

ALTER FUNCTION [dbo].[udfDailyKwHrUsage] 
(	
	-- Add the parameters for the function here
@StartDate  datetime
	
)
RETURNS TABLE 
AS
RETURN 
(
	-- Add the SELECT statement with parameter references here
	SELECT tblPOWER.P_DATE, tblPOWER.P_KW, tblMETER.M_DESCRIPTION FROM tblPOWER INNER JOIN tblMETER ON tblPOWER.M_NUMBER = tblMETER.M_NUMBER WHERE (tblPOWER.P_DATE > @StartDate) AND (tblPOWER.P_DATE < dateadd(day,1,@StartDate))
)

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