Hi,
answering your questions are more difficult as it would seem...
Your project could be as simple as a one line javascript code with a counter, or as complex as an individual dynamic counter, using sprite images, instead of letters and database/session/cookie informations from each user.
We could use freelancer as example. Lets say I start on freelancer with 100 minutes and each time I place a bid, I get 1 minute, each time I finish up a project, I get 500 minutes. If my time is running out, my account will be locked and I have to buy extra time with real money... In order to make this work, a client side counter is not enough. Informations about the time need to be stored in a database, so time can continue run down, no matter the user is online or not.
But as I said, this is would be a real complex situation and an implementation like that would cost far more than 30 bucks.
If you consider my bid, please let me know and give me additional informations about what exactly you need, eg. what you mean with "could remove or add time".
Please ask me anytime if you have more questions.
Kind regards,
Thomas