Just to keep in mind: the column was already there and is also used to measure/log the time elapsed between inserting/scheduling a job and the job being locked (see [QC::Queue#lock](https://github.com/severin/queue_classic/blob/enqueue_at/lib/queue_classic/queue.rb#L60..L64))...

I would have renamed the column to `scheduled_at`, but `not_before` would also be okay.