Yet more vicissitudes of rank

The show: JAG
The episode: "Second Acts," first aired April 1, 2003, now on Disc 3 of the Season 8 boxed set.
What happened: Stuart Dunston of ZNN shows up at Camp Stronghold Freedom to do a story on the Seabees when a bunch of terrorists show up and start shooting up the place. With little hesitation, Petty Officer Steven Wilson uses his tractor to defend his unit and thwart the terrorists, saving the lives of Stuart and his news crew. Jennifer Bruder recognizes Steven Wilson as Matthew Divine, a man who supposedly died in the World Trade Center on September 11, 2001, together with her husband. At first Wilson refuses to tell Jenny what happened to her husband. It takes an administrative separation hearing for Wilson/Divine to admit the truth: With Jenny's husband, he had run a dishonest business operation. On September 11, he promised not to tell Jenny what had happened. He bought a new identity and joined the Navy Seabees.
What doesn't quite add up: Assuming that the events of this episode take place on or around the original airdate of April 1, 2003, this barely gives Steven Wilson 18 months to go from recruit to Petty Officer Third Class. Even if Matthew Divine had prior military experience, as Steven Wilson he couldn't use it to skip E-1. It's my understanding that in the Navy, you need 9 months of service at E-1 and the unit commander's approval to get promoted to E-2 (in the Marine Corps, six months at E-1 with a clean nose are sufficient for promotion). To get to E-3, the Navy requires 9 months at E-2, a couple other qualification and again the unit commander's approval. That's 18 months right there. At that point, if the Navy is hurting for people to do your job at the E-4 level, you might get promoted to E-4. (Doing something heroic like Wilson does at the beginning of the episode might also help). But in the Marine Corps, you'd need still 6 more months of service to attain E-4, and yet another 18 in the Air Force. It's in the Army where you have the best chance to climb up to E-4 in less than 18 months. I don't know if the Navy's rules apply equally to the Seabees, but it seems to me that it would have been much more realistic for Wilson to start the episode at E-3 and then be promoted to Petty Officer in recognition of his actions at Camp Stronghold Freedom.
And another thing: One of the datelines identify Divine's administrative separation hearing as an "Ad Sep Hearing." The Navy likes to abbreviate things, but "administrative separation" is usually shortened to "admin sep."