After enabling your post as a Swift Post, you need to set when you’d like it to run.
If you are creating a new post and would like it to opporate as a Swift Post, you can select and future start date and time.
For example, if you were creating a new post on July 1, 2016 and would like it to from July 14, 2016 – July 20, 2016, you would enter those dates and then click “Publish” like normal. The post you created would run like normal until July 4th when it would become a Swift Post and start serving in the locations you set it up to serve.
This can be done with any post, no matter how long ago it was created.