There may be situations where you want to publish a post with a date in the past. On first sight WordPress does not have such a function. If you look closer you find on the page “Add new post” in the metabox “Publish” the following entry:
If you click on Edit a field appears in which you can enter the desired publishing date:
You can not only choose a date in the future but also a date in the past. After you have entered the date you should not forget to OK because otherwise the new date isn’t used when you publish the post:
If you press Publish the post will be published with the date set above:
Update: Monika Thon-Soun advised me that this practice may not be suitable if you’re focusing on search engine ranking since the subsequent entry may confuse Google et al.