The problem with the 30-day default window is that if you have a longer sales cycle, you might be missing the credit for a conversion action. It’s important to understand that a credit is given for the impression prior to the click that led to the conversion. So if you’re looking at a 14-day window, but the click or impression prior to the click happened 15 days ago then you might not see the conversion that just happened. I know, that can be quite confusing, and that’s why sometimes conversions are missed.
So this is the help document that is showing the time of the impression preceding the click that leads to the conversion. That’s when that conversion takes place. This plays into looking at your reports, and this also plays into choosing the conversion window.