If you try to edit a job that is more than 30 days old, you will see a Job Posting Warning. It asks whether you would like to refresh the job and transfer your candidates, or keep editing the current one.
Refreshing a job is not the same as editing it.
Editing a job
Editing lets you update the posting without changing its original post date. The job keeps the date it first went up, so if you edit a job that is over a year old, the post date will still show as over a year old.
Choosing to edit
If you select Edit Current Job, you will see the warning again and can either refresh the job or continue editing it as it is. Pick whichever fits what you are trying to do.