time_block.rb

def time_block(&block)
  raise "No block given" unless block_given?
  start_time = Time.now
  Thread.new { yield }
  Time.now - start_time
end

def repeat_every(seconds, &block)
  raise "No block given" unless block_given?
  begin
    time_spent = time_block { yield }
    sleep(seconds - time_spent)
  end while time_spent < seconds
end

repeat_every(300) do
  puts "Start checking: #{Time.now}"
  puts "End checking: #{Time.now}"
end