This looks good. A couple of things I'd suggest:

1) Instead of making it a prototype function, make it property and calculate it once when the model is being made. This is more efficient because the value isn't going to change, so you don't need to re-run the entire function on every digest cycle.

2) You can use moment.js to simplify the math. Moment.js is already installed in the repo, so you can access it by injecting $window then using $window.moment. Here's the documentation http://momentjs.com/ it's a really awesome library. Also the api for subtracting time: http://momentjs.com/docs/#/durations/subtract/