Yes. We track service time in days. It is viewable by hovering over the ML Years value on a player's profile. Each day a player is on the big league roster during the regular season (even if he's on the DL), he accrues a day of service.
In addition, we track the # of days a player is on optional assignment on a minor league team. If a player is on optional assignment less than 20 days during the season, this total is added to his big league service time.
At the end of the season, if a player has 172 days or more (including any optional service days < 20), he is given a year of big league experience. Excess days or days under the 172 day minimum roll over into the next season. These years are what dictate arbitration eligibility and free-agent eligibility.
It is important to note that a player registers days on optional assignment only after the minor league schedule has begun. There is typically a 1 or 2 HBD day difference between when the big league schedule starts and when the minor league schedule starts.