The expiry date is the only certain date in the hiring process and gives applicants a definite time frame. Other dates are just targets. In WPJO you can add an Expiry date to the job posting easily. You can make it visible to the candidates or hide it from them. Once this date passes, the job listing is automatically ended (if enabled in the settings).
Using the awsm_job_expiry_details_content filter hook, details of job expiry can be modified easily. The filter hook can be found in wp-jobs-openings.php location