Make impossible to set a Departure date if Arrival date is not set

Make impossible to set a Departure date if Arrival date is not set

Hi.
I am using the jQuery datepicker to get Arrival and Departure dates, with success.
I would like to improve it making it impossible to set a Departure date if Arrival date is not set. When the Departure input field is selected by the user but if the Arrival date is empty I would like to force it opening the datepicker in the Arrival input field.
Can give me some guidance to achieve it?

Thank you very much.