loading values in asp list box using javascript

I have 2 asp drop down list one for month and one for days.  The month I have hard coded.  But the days I want to load dynamically based on the month.  So here is my code:
<asp:DropDownList ID="_DOBMonth" runat="server" onchange="isDOBCorrectFormat();" CausesValidation="true" Width=&qu ...