cookie expiration

i want to set the cookie before the login lik:if the user fails to login by typing wrong user id or password in 15 minutes then he should be restricted for the nest 15 minutes to login..

using Javascript i have set a cookie i want that if the use开发者_运维知识库r has exceeded 15 minutes from his first trail then he should recieve an alert or message on window(just like gmail or yahoo)

any example help please its urgent

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store the user machines IP and a boolean field- to allow,--> in Database. retreive it and restrict them.(just Logic).