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

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