X minutes past the hour

[ph...@philsmy.com]

We want to spread our jobs out and have one run at (for example) every hour, but at 18 minutes past the hour. I don't know if whenever supports this as an 'at' parameter:

every 3.hours, at: '18m' do
  runner "MyClass.runnit", :output => nil
end

Is there something handy like this or do we need to do something like in the question on how to do every 90 minutes?