My team is a shoo-in for the playoffs and the number one seed. I have a very strong lineup but I have a very strong 3B bat in AAA that will be starting for me next year. I didn’t call him up this year because he’s blocked and I didn’t really need him. If I want to call him up on the last day of the season and add him to my playoff roster, how will that impact his service time? I was planning to keep him in the minors for the 20-game window next season.

Service time is calculated based on the regular season, so the postseason will have no impact on that. To manipulate service time next season, you will just need to wait another day (so wait 21 games instead of 20 if you bring him up after Game 161).

As for adding him for the playoffs, is he already on your 40-man roster? If not, you might run into an issue there. In real-life MLB, I know you can do add a player who isn't on your 40-man for the playoffs, but not sure if it works the same in HBD. One thing to do is to make sure to add him to your playoff roster since he wasn't up when rosters expanded. Don't just think he is on there because you promoted him to the majors. The playoff roster add is an additional step at this point in the season. Side note: You might want to put him on the 40-man while he is in the minors and see if you can add him to the playoff roster before promoting him. That won't affect anything for next season.

You don't even need to call him up during regular season. Wait until the off day before playoffs start and call him up. But looks like your team has a 1st round bye so you've got even more wiggle room. Just as long as he is on the playoff roster before the first game of a series.

Side note: You can always add/remove guys from the playoff roster between a playoff series (just not during a series - and only as long as they were initially eligible for the playoff roster, i.e. they are on the 40-man roster before roster expansion)

Appreciate the answers guys!