Javascript Interval Counter

I need an entirely browser-based counter/timer. The script will be placed into a website. The script needs to: Look at the current date and time on the machine, then work out the time that has passed since 12 noon, December 19, 2016. It then needs to work out how many 'intervals' (units of 6171 seconds) have occurred between the times, and display the result. You don't need to make it look pretty but it does need to be functional and fully working. I will need to see a working plain-text prototype.