You can use asp compare validator for this purpose.. for example if you have two textboxes i.e
txtDateStart
&
txtEndDate
and you want to validate them user can not enter end date which comes before start data.. mean 7th june should not be start date if 6 june is end date.
Following is the sample code.
<asp:CompareValidator ID="ValidatorID1" runat="server" ControlToCompare="txtStartDate" ControlToValidate="txtEndDate" ErrorMessage="Start Date must be before End Date" Operator="GreaterThan" Type="Date" Display = "None"></asp:CompareValidator>
Hope it will help.
Regards, :)