Execute Postgres Function in VBNet

Question

I want to execute my postgres function having date as parameeter in VBNet and show the result in datagridview.Below is my code Please tell me where i am getting wrong.I think i am writing wrong command to execute function.I know this might be wrong way but i am using first time postgres function in VBNet

Imports Npgsql
Public Class Form1
    Dim connstring As String = String.Format("Server=127.0.0.1;Port=5432;" +
  "Username=xxxx;Password=xxxx;Database=EMS_Demo_db;")
    Dim conn As NpgsqlConnection = New NpgsqlConnection(connstring)
    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        Try
            conn.Open()
            Dim Command As NpgsqlCommand = New NpgsqlCommand("SELECT * FROM lastsevendays_sp ' + DateTimePicker1.Value + '", conn)
            Dim reader As NpgsqlDataReader = Command.ExecuteReader()
            Dim data_Table1 As New DataTable
            data_Table1.Load(reader)
            DataGridView1.DataSource = data_Table1
            reader.Close()
            conn.Close()
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub
End Class