即使用户不在页面上，也要跟踪计时器

At the moment, I have a simple signup and login PHP system going, I want to achieve a little timer which the user can set up for their personal use, I was wondering if they could type in something like 12 hours and then a timer would count down until that timer ran out and they could start it again, I was thinking the timer could keep going even when they don't have the page loaded, so I was thinking it needed to be stored in a mySQL database, I am just having great difficulty with this and I hope I'm not asking for too much.At the moment, I have a simple signup and login