Slack has integrated with Google calendar allowing post reminders inside slack channels.
According to the Verge, the users can set any calendar to post events to any channel at set times.
It is easy to reminders in this app as it would slot right into the timeline of messages.
The app can remind on both a group and personal level.
Also, the users can add their personal calendar and Slackbot will happily directly message the users reminders instead.