@jlevy: you can still do it with crontab 
Let’s explain it with your example: user may trigger something like "send this message tomorrow at 5pm.".
Possible solution: You have a GenServer that asynchronously saves messages with specified date in database. After that you are generating crontab entry and update its configuration. crontab calls task which fetches required data from database and send it.
The 1% edge-cases are for example plug-in system when code is loaded dynamically and therefore there are no mix tasks to run. Of course someone could find a good hack for it, but we should keep things as simple as possible, so in most cases crontab should be enough.
