http://project.ioni.st/post/2288#snippet_2288

gistfile1.rbx

class Date
  def at_some_point(options = {})
    start_hour = options[:between]
    end_hour   = options[:and]
    if start_hour && end_hour
      hours = rand(end_hour) + start_hour
    else
      hours = rand(24)
    end
    
    minutes = rand(60)
    seconds = rand(60)
    Time.mktime(year, month, day, 
                hours, minutes, seconds)
  end
end

class Fixnum
  def am
    self
  end
  
  def pm
    self + 12
  end
end

Job.schedule task, :to_run => 
  Date.today.at_some_point(
    :between => 1.am, :and => 7.am
  )