compare hours and minutes if dates are equal

I want to make sure that users can't submit a case that ends before it starts.
I have two textboxes for startDate and endDate. Those I validate with comparevalidator Operator="LessThanEqual". This works fine. My textboxes uses ajax calendarextension from Microsoft.

<asp:CompareValidator Display="Dynamic" ID="CompareValidator2" runat="server" ControlToCompare="endDate"
ControlToValidate="startDate" ErrorMessage="End date must be after start date" Operator="LessThanEqual"
Type="Date"></asp:CompareValidator>


But I also got four dropdowns with hours and minutes. endHH, endMM and startHH, startMM.
I want to compare the hours and minutes if the dates are equal.

How can I compare this?
/Markus

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