Usually my suggestion would be to have a datetime field on the article to specify when it should be published but since you are fetching the data from an external source you would have to inject a date automatically 
You could fetch the previous date using something like strapi.query('article').find({ limit: 1, sort: "created_at:desc" }) then use something like moment.js to add additional time for the new article.
Then your cron could check for articles to post. Interesting flow for sure, we do have a guide on this here but it doesn’t have anything along the lines of fetching the articles automatically: Scheduled Publication - Strapi Developer Documentation