An example showing time now slipping, as long as job takes less time than interval

now_and_every.rb

require 'async'
require 'async/notification'

def do_a_thing_lasting(duration)
  puts 'hi'
  sleep duration
  puts 'bye'
end

def now_and_every(interval)
  Async do |task|
    notification = Async::Notification.new
    task.async do
      loop do
        notification.wait
        yield
      end
    end

    loop do
      notification.signal
      task.sleep interval
    end
  end
end

now_and_every(2) { do_a_thing_lasting(1) }