<ed:Editor id="editor" runat="server" Appearance="custom">
    <Buttons>
        <ed:Method Name="Undo" />
        <ed:Method Name="Redo" />
        <ed:HorizontalSeparator />
        <ed:Method Name="ClearStyles" />
        <ed:HorizontalSeparator />
        <ed:Select Name="FontName" />
        <ed:HorizontalSeparator />
        <ed:Custom OnClientClick="insertDate" ImageName="ed_date_n.gif" ToolTip="Insert current date" />
    </Buttons>
</ed:Editor>
...
<script type="text/JavaScript">
    function insertDate(editor)
    {
         editor.InsertHTML((new Date()).toLocaleDateString());
    }
</script>