Buffer has some default posting times but they don't let you post at random times. You'll have to create them, or better yet, utilize a service like Zapier.
Buffer Social Media Management Software for Growing Brands
How to Save Time While Scheduling Content for Social Media
Scheduling Instagram posts, reels, stories, and reminder notifications - Buffer Help Center
Saving and scheduling draft posts - Buffer Help Center
How to Save Time While Scheduling Content for Social Media
How to Save Time While Scheduling Content for Social Media
Tracking geoprocessing workflows with QGIS & DVC
How to Save Time While Scheduling Content for Social Media
Saving and scheduling draft posts - Buffer Help Center
How to Save Time While Scheduling Content for Social Media