I'd recommend using a timer object. You can set it to execute a callback function every second. Within that callback function, you can update your countdown variable. It seems like the thing to try first in your scenario. Here are a couple links that include a basic example on how timer objects work:
As for your question about what data type to use, a datetime array should be fine. Alternatively, you could convert the datetime into the number of minutes or seconds remaining if that is all you need from this variable. Hope this helps.
