Countdown timer AJAX and PHP

[v0idmp3]

Hi

I have a little problem. I develop browser game and I dont know how to build countdown timer. For example...When player click on "Build Something", I need to show him a countdown timer, from start time to some time in future, and countdown difference from start and end time. And than, when timer reach zero, I need to put some data in database. I think a lot about that, and I realized that I must have in my database start and end time, but I dont know how to calculate remaing time when player logout from game, and than log in again. How to show him how much time was spend when he was offline? If you know some tutorial about Ajax/JS countdown timers, I will be very thankful. I searched google, but I didnt find what I need. I'm new to ajax/js and I really dont know how to make that.

Cheers

[probi]

Not sure about how to code it but would it be a good idea to have an end time in the database and when the user logs in you read the end time and then calculate the time left between the current time and the end time?

[v0idmp3]

So, I need third field in db, like "user_current_time" to populate with time when user log in and than calculate difference between "user_current_time" and "end_time"?