Adjust new Date fixture syntax [Firefox Bug]

I noticed while following along with the screencast that the dates were invalid. Cloning the repo, it seems to not be working as well in Firefox. The string date passed into `new Date()` must be compatible with `Date.parse()`, [according to the MDN JS docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/parse).

This PR adjusts the way that the date is created with a format that is compatible with Firefox (and still works in Chrome).