I've used the scheduler, with great success, many times.

Last night however I scheduled 3 posts around 8:50PM EST using the account "JarvisBot" to "marvelstudios@lemmy.world".

Unfortunately each post was posted ~3 times each, causing some spam. The also never disappeared from schedule.lemmings.world/post/list so it seems like they were recorded as failures? I deleted them manually just in case.