We have a return date on the kit to help remind you to send back your kit as soon as possible. If you have not made your impressions yet, your kit is fine. It won't be 'bad' if you take your impressions a few weeks or months later. 

However, once you make the impressions, you need to ship them back right away, because your teeth might shift over the time span of a few weeks or months, so we can't make new aligners on 'old' impressions (or they might not fit properly). 

That's why we encourage you to make your impressions soon after you get your kit and ship them back as soon as you're done. And why would you wait anyway? The sooner you get your kit in, the sooner you could be on your way to your new smile.