The annual Assateague gate pass is good for 12 calender months, no matter what month it was purchased. This pass will activate the gate allowing you to bypass the tourists waiting in line to pay.
The National pass will not open the gate, you have to wait in line.
Obviously, the Assateague pass is good only at Assateague Island. National pass, all parks.
I have had both passes in the past because I didn't want to wait in line but still wanted the National for the others. I have found that I personally need only the National because I don't hit the island when there is a line at the gate. Heck, most of the time the booth isn't even open yet.
Adjust your need accordingly.
*added* The annual national pass goes from 01/01 to 12/31, no partial pay.
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