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i have no idea as i have never personally found the use for such a thing
however if there is only a one second difference between what the server is, and what the client is, unless your working for the atomic timing agency it wont matter
Perhaps if the [Start] triggers a function that will update a database/file once/sec, in conjunction with a 3rd part software (read: cron-job?) or similiar.
Flash or Javabean/applets is also something I firts thought of. Don't think you will be able to do this in pure PHP. Try the guru's in the "Client Side" part of the forum...