If your CalendarExtender is half hidden by divs you should be able to fix it by one or both of these solutions:
In the css, give it a high z-index:
.ajax__calendar_container
{
z-index:1000
}
Make the div position absolute:
<div style="position:absolute;height:auto">
<div id="requiredBlockCompletedDate" runat="server" />
<asp:TextBox ID="txtCompletedDate" runat="server" CausesValidation="True" MaxLength="10" Text='blah'></asp:TextBox>
<asp:ImageButton ID="imgCompletedDate" runat="server" OnClientClick="javascript:return false;" SkinID="CalendarImageButton" />
<ajaxToolkit:CalendarExtender ID="CalendarExtender3" runat="server" CssClass="ajaxToolkit-CalendarExtender" Format="dd/MM/yyyy" PopupButtonID="imgCompletedDate" TargetControlID="txtCompletedDate">
</ajaxToolkit:CalendarExtender>
</div>
