What is easiest way to validate Start and End Date fields

What is the easiest way to validate start and end Assign Date fields where a validation error is thrown if end date is earlier than start date. For example:
Start Date 5/26/2015
End Date 5/26/2014
The error would be End Date cannot be earlier than Start Date.
 
Any help is appreciated.
 

<td style="white-space:nowrap"> <asp:TextBox runat="server" ID="txtAssignDateStart" Width="85px" MaxLength="10"></asp:TextBox> <asp:ImageButton ID="ibtnAssignDateStart" runat="server" AlternateText="Assign Date Start" CausesValidation="false" ImageUrl="~/images/calendar.gif" />&nbsp; to&nbsp; <asp:TextBox runat="server" ID="txtAssignDateEnd" Width ...