How service time accrues
A full service year is 172 days on the active roster or major league injured list. A season is 186 or 187 days long, which creates the gap teams have historically exploited: hold a player down for the first two-plus weeks and he finishes the year at 171 days, one day short, pushing free agency back a full season.
Service time is written as years.days — 2.145 means two years and 145 days. Six full years unlocks free agency. The current CBA added draft-pick incentives for teams that promote elite rookies early and full-year credit for rookies who finish top-two in Rookie of the Year voting, which has softened but not eliminated the practice.
The three-year arbitration window
Years one through three are pre-arbitration: the team sets the salary, typically at or just above the league minimum, with a pre-arb bonus pool distributing extra money to the best young performers.
Years four through six are arbitration. Player and team each file a number; if they do not settle, a three-person panel picks one figure or the other — no splitting the difference. The comparison set is players with similar service time and similar traditional statistics, which is why arbitration still rewards RBI, wins and saves more than modern analysis would.
Super Two
The top 22% of two-plus-year players by service time qualify for a fourth trip through arbitration. The cutoff moves every year, usually landing around 2.115 to 2.130.
A Super Two designation can add $5 million to $15 million of career earnings for a good player, and it materially changes his trade value: four arbitration years instead of three means higher projected salary across the same control window.
Why control years dominate trade value
Surplus value is production minus cost, so the cheapest years are the most valuable ones. Roughly speaking, a good young regular generates most of his lifetime trade value in his three pre-arb seasons.
That asymmetry explains the entire deadline market. A contender trades future control for present wins; a seller converts two months of a veteran into six years of a cost-controlled player. Neither side is being irrational — they are pricing different currencies.
Options, the 40-man and Rule 5
Each player has three option years, during which he can be sent to the minors freely. Once options are exhausted, he must clear waivers to be sent down — which usually means losing him.
Players signed at 18 or younger must be added to the 40-man roster after five minor league seasons; players signed at 19 or older after four. Leave them off and any club can select them in the Rule 5 draft, provided the selecting team keeps them on the active roster all of the following season. Every November, the Rule 5 deadline forces teams to make real decisions about fringe prospects, and those decisions frequently drive December trades.