Scheduling with FollowPlanner works with a weekly schedule, instead of manually or randomly specifying the posting times.
On the Post Schedule page you will find a calendar, the posting calendar. To create your first posting period, just click on the table at the day you want. Each block is equal to a post (they are picked from "Upcoming posts" at the Media Library, in order from left to right) and it will be published at a random time between the start time and the end time, on a weekly basis – this allows you to set up the schedule once and forget about it.
You can resize each block by clicking their top or bottom and dragging. To remove a period, just click the X on the top right corner.