VS 2008 Dropdownlist Selection Disables / Enables Another Dropdownlist

I have a dropdownlist on my page with dates such as 1/1/2013, 2/1/2013, etc.

Then there is a second ddl.

If the date in ddl #1 is before 1/10/2013 I want to enable ddl #2.
If the date in ddl #1 is on or after 1/10/2013 I want to either disable ddl #2 or hide it (whichever's easier).

Disabling ddl #2 based on the selected date is a new requirement so I am enhancing the existing code. Obviously I want to do this without a postback, but I don't know what is the best way. I am trying to understand what an AsyncPostBackTrigger is because this is the existing code:

Code:
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:dropdownlist id="EffectiveDateDropDown" runat="server"></asp:dropdownlist>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="QuoteType"
EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>

It wants to do something when the selected index is changed? This exists in the code-behind, but I set a breakpoint and never hit it:

Code:
protected void QuoteType_SelectedIndexChanged(object sender, EventArgs e)
{
try
{
}
catch (Exception err)
{
this.PublishError(err);
}
}