Javascript and PHP countdown Timer that displays the same for everyone

I have the script needed to generate a countdown timer that has a start and resume button. What I’m attempting to do is to have Start, stop, and reset buttons on one page that controls the timer on the other page.
So if user X visits page.html they will see a timer that is at 0.
Admin X visits admin.html where they see the timer at 0 but they also have a start, stop, and reset buttons. When the Admin clicks a button, the timer on page.html starts to countdown. If another user visits the page while the timer is counting down, they will see where the timer currently is at. If anyone has any code ideas, other answers on this site I can reference, or the code that I would need, I would be very thankful.

PHP MYSQL dynamic select box

I am trying to create a search box where the option selected from ‘box1’ populates the options available for ‘box2’. The options for both boxes are given from my MYSQL database. My problem is that I do not know how to perform a query based on the first query without refreshing the page which would be tedious and annoying.