A script I use for this occasionally:

  #!/usr/bin/env ruby
  done = Time.now + ARGV[0].to_i
  while(true) do
    sleep(10)
    `say "Time's up!"` if Time.now > done
  end

Or the alarm clock version:

  #!/usr/bin/env ruby
  require 'time'
  done = Time.parse(ARGV[0])
  while(true) do
    sleep(10)
    `say "Time's up!"` if Time.now > done
  end