Save the future posts to the database with the required publish date/time.
Run cron every $X minutes where $X is the smallest interval you allow (ie if you force publish date to be rounded to 5 minutes, run cron every 5 minutes) and check for "future posts" where the publish date <= now(), and take action.
This is pretty basic stuff and that you want to offload it to a 3rd party by capturing all the details into a request to be made back to your app is a huge red flag IMO.
