Your question is confusing. Day 1 starts when you swear in. You are E-1.
Promotions in the Marines to E-2 and E-3 are automatic based on being well-qualified.
Promotions to E-4 in the Marines are competitive.
In the Marines, automatic promotions is true only for promotions to the ranks of E-2 and E-3.
The promotion criteria for promotion to the ranks of E-2 to E-3 are:
Private First Class (E-2) - Six months Time-In-Service (TIS) with six months Time-In-Grade (TIG)
Lance Corporal (E-3) - Nine months TIS and eight months TIG.
You make Private First Class 6 months after you enlist. Remember, you are out of "boot camp" after 12 weeks (3 months) so don't base promotion on "boot camp" time. Base it on your time-in-the service and your time-in-grade.
SO: don't confuse yourself comparing Promotion to PFC based on "boot camp." Base it on the first day you enlisted! Remember, you could get set back in boot camp a week or two but you are still going to make PFC when your time in service is 6 months!`
Then, 9 months after you make Private First Class (E-2) you are promoted to Lance Corporal (E-3). By that time, you have 15 months in the Marine Corps.
Promotions to E-4 and above are competitive, and are based on specific vacancies within MOS's (jobs).
The Marine Corps takes the number of "slots" they have for each enlisted rank, above the rank of E-3, and allocates them to the different MOS's (enlisted jobs). In other words, MOS 123 may be allowed to have 5,000 E-4s at any point in time and 2,000 E-5s, and MOS 456 may be authorized 7,000 E-4s, and 5,000 E-5s (as a general rule, the higher the rank, the fewer positions there are).
In order to promote someone (above the rank of E-3), there must be a "vacancy." For example, if an E-9 retires in a certain MOS, that means that one E-8 can be promoted to E-9, and that opens an E-8 slot, so one E-7 can be promoted to E-8, and so-forth. If 200 E-5s get out of the Marine Corps in a particular MOS, then 200 E-4s can be promoted to E-5.
The Marine Corps has 154,348 enlisted members on active duty. Here's how it breaks down, by enlisted rank:
Private (E-1) - 9,671 (6.3%)
Private First Class (E-2) - 20,625 (13.4%)
Lance Corporal (E-3) - 43,141 (30.0%)
Corporal (E-4) - 29,578 (19.2%)
Sergeant (E-5) - 23,019 (15.0%
Staff Sergeant (E-6) - 14,794 (9.6%)
Gunnery Sergeant (E-7) - 8,801 (5.7%)
Master Sergeant/First Sergeant (E-8) - 3,306 (2.1%)
Master Gunnery Sergeant/Sergeant Major (E-9) - 1,413 (0.9%)
In the Air Force we don't do it that way.
ALL career fields (MOS in the Marines) get the SAME percentage of promotions.
Example: AF determines how many people will be retiring or not reenlisting at any given promotion cycle. Then, it assigns a percentage for promotion. It makes no difference what job you have. All have the same percentage. If the AF feels that we can only promote 12% of E-4 to E-5 than EVERY AF JOB category gets the same percentage based on two promotion tests taken annually and other factors such as time in grade and time in service points and decorations, if any.
Remember, AF automatically promotes to E-4, unlike Marine Corps who makes you physically meet a promotion board. AF doesn't make people meet promotion boards. Our promotions are based on tests and computerized grading and lining up everyone who is eligible from #1 on the top to the very bottom person whose points are the lowest. Then, if 8% promotions are announced than the computer goes down the list and selects that top 8% for promotion. If two people are tied with the same score they both get promoted. The persons below the cut-off line also are notified of their standing so they can study harder for the next annual promotion.
For E-8 and E-9 our complete personnel records jacket meets a sitting board of 3 officers who examine your total military record and score your record with points. All 3 board members score must be with 1/2 point of each member or your record goes to a different table with 3 different board members to re-score you until there is agreement.
That is like 3 judges at a boxing match who each score the combatants. But, in AF senior promotions there is no "split-decision." All have to be in agreement.
Senior Master Sergeant, USAF (Ret.)
Previous military experience of 27 years: 1961 - 1989