I have the following code:

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="DurationPicker.ascx.cs" Inherits="Permias.DurationPicker" %>
<%@ Register TagPrefix="Club" Namespace="ClubSite" %>
<div class="controlblock">
    <table>
        <tr>
            <td>
                Start Date:
            </td>
            <td>
                <Club:DatePicker ID="dp1" runat="server" />
            </td>
            <td>
                <Club:TimePicker ID="tp1" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
                End Date:
            </td>
            <td>
                <Club:DatePicker ID="dp2" runat="server" />
            </td>
            <td>
                <Club:TimePicker ID="tp2" runat="server" />
            </td>
        </tr>
    </table>
</div>

In my code behind I have

public System.DateTime startDateTime
    {
        get
        {
            return dp1.SelectedDate.Add(tp1.SelectedTime.TimeOfDay);
        }
        set
        {
            dp1.SelectedDate = value;
            tp1.SelectedTime = value;
        }
    }

However dp1 is underlined in red, which means it can't find dp1.. why is this?