Vba code for validation rules to prevent users from selecting th

Hi, I am developing an Access 2007 database and would like to write code for a validation for two combo boxes on a form. One combo box has a drop down list of starting times and the other combo box has a list of ending times. I entered a validation rule and text message on a form, but whenever I selct a time a pop up message keeps displaing and prevent the user from making a selection, so they have to close down the database and re-open it, as you can imagine this can be frustrating for the user to do. What I would like to do is prevent the user from selecting either an end time that is earlier than a start time, and a start time that is later than an end time. Is this possible to do? If so, how can this be done? If you have any examples that would be very helpful. Thanks for your solutions. I look forward to your response.


Astley