The name Postcron suggests the answer.
CRON is a Unix/Linux based tool for scheduling events, using complex repeat timings.
Publishing to media accounts just means connecting so some API, like Twitter or Facebook or Meetup or whatever, then posting some text.
You can do this off your desktop using the curl tool.
The reason these services are normally paid, is because handing all the API interactions + API changes over time requires time + development dollars.
You can do this for free... free as in you spend no cash + you'll have to pay in massive amounts of time to manage all this yourself.
Custom social media posting requires a huge amount of time to keep running, due to constantly changing API details.