Lately, MLB teams have been exploiting a rule that allows them to keep stellar prospects for another year. It happened with the Blue Jays and Vladimir Guerrero Jr. It also happened with the White Sox and Eloy Jimenez. But was is service time and what allows MLB teams to do this?
Players receive service time for every day that the player spends on the IL or on his team's active roster. That includes off days. The MLB's regular season is 187 service days long. A player needs 172 days spent on the IL or the active roster for a year of service time to count. After 6 years of service time, the player becomes a free agent.
In Vladimir Guerrero Jr's case, he was brought up 15-16 days after the MLB season begins. Therefore, he doesn't get a year of service time, and the Blue Jays got a full year out of him without it counting towards his six years with the Blue Jays.
Over the offseason, Kris Bryant filed a grievance against the Cubs. He lost, but the damage had been done. Bryant's case was that in 2015 Spring Training, he hit .425 with 9 HR and 15 RBI in 40 AB. But still, he was optioned to AAA to start the season. After about 3 weeks, Bryant was called up. Therefore, when his contract expires he will have 6 years, 171 days of service time.
Although service time is a broken rule, there are no better options to replace it. Because of this, teams will likely exploit this rule well into the future.