In recent years, you've probably heard the term "service-time manipulation" bandied about in baseball circles. Perhaps you are uncertain about what exactly service-time manipulation is. As with all matters of import, we are here for those with nowhere left to turn. That is, let's take a few revolutions of the second hand to explain what service-time manipulation in MLB is and why it matters.
In keeping with the injunctions of ancient sacred texts we shall do this in FAQ format with assistance from an imaginary interlocutor with an abiding interest in tedious business practices. Forthwith!
First of all, what's service time?
In MLB, "service time" is the term for how much time a player has spent in the majors, either as a member of the active roster or on the injured list. It's measured in days.
A typical MLB regular season -- it's worth emphasizing the word "typical" in light of the ongoing COVID-19 pandemic -- spans 187 days, and players must be on the 26-man roster or injured list for at least 172 of those days in order to accrue a full year of service time (or 172 across multiple seasons). Stated another way, players can be off the active roster/IL for up to 15 days during a season and still earn a year of service time. Also, players can't earn more than 172 days of service time in a season even if they exceed that number of days on the roster or IL.
Service time matters because it's in essence the "clock" for players when it comes to earning higher salaries via arbitration eligibility and free agency. Players are eligible for free agency after six full years of service time. A player with five years and 171 days of service time? His free agency gets delayed by a full year because he didn't satisfy the 172-day cutoff in that sixth year.