My guess at a clarification is this:
In the calendar year 2017 - everyone gets to use the national parks for free.
If you buy a pass in 2016, it lasts until the same date in 2018, to give everyone the full 12 months of paid value. If you buy one in June of this year, you are paying for the 6 months remaining in 2016, and the first 6 months of 2018.
You won't be able to go get a pass for free in June of 2017 that lasts until June of 2018.
There isn't really extra incentive to get one now - becasue it will last until 2018. If you normally would get one, you will still want to get one.
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