[bigtones]

Looks like they needed to have built-in support for time based scheduled execution of jobs at a specific time in the future. Rabbit MQ does not have that (that I'm aware of) without doing some horrible hacks around one queue per message.

[kondro]

ActiveMQ's had that for a couple of versions though. Another highly performant/available, atomic, consistent, durable and mature queueing server.

[cyounkins]

Correct. Celery's implementation dequeues but doesn't acknowledge the task, holding it until the timer is reached.