Option one is to keep your original 'end date'... you'll go out when you can. (your mission clock continues to tick as normal)

Option two is to go out when you can (within 18 months)... you will get a new 'end date'. (your mission clock goes on hold until you go out again)

There is NO option to be released permanently. It's option 1 OR option 2.. there is no option 3 for early release.