ruby-on-rails - Clockwork won't run a job in the closest full hour

Question

I'm using clockwork for running a job every full hour (**:00), like this:

every(1.hour, 'auto lock', :at => '**:00')

But if a add an item for the job, for example at 17:50, the job will run for this item only at 19:00 and not at 18:00 as expected.

What could be the problem?

Thanks.