For a lot of the tips (don't repeat keys, use ISO8601 dates), most JSON libraries should follow these conventions. So as long as you aren't hand-generating your JSON, you'd need to do very little work to conform to these guidelines.
Week numbers are a very common way of talking about time in some locales.
For instance in my native Sweden, it's very common when e.g. scheduling things at work to talk about which week something will happen.
Sites like http://vecka.nu make it easy to check the current week number (the domain name translates as "week.now") whenever you have internet access, paper calendars include it, and so on. It's really very common.
