Evaluate Open And Closed Dates

I have two date fields that I am using a form to show the open and close date of an investigation. The opened date field is controlled by a textbox elsewhere on the form, but the date closed is controlled by the vbCalendar control. What I am trying to do in coding is to evaluate whether or not the closed date is before the open date. If the date closed is before the date open I would like a message box to open that prevents the user from moving forward and reminding them that the date closed is earlier than the date open. Is it possible with the vbCalendar option being able to change its focus continually.