'Which planet's dasha' alone can't tell you the outcome.
You learned Dasha as a '120-year timetable.' Now let's look at how to actually read it. This is where beginner and intermediate readers part ways.
First, Your very first Dasha doesn't start fresh from the beginning the moment you're born. Depending on how far the Moon had already moved through that Nakshatra at the moment of your birth, your first Mahadasha starts already partway through. That's why the remaining length of that first Dasha differs from person to person.
Second, there are layers. Inside the biggest, the Mahadasha (6–20 years), there's the Antardasha, and inside that, there's the Pratyantardasha again. The further down you go, the more finely you can read the timing. Here's a common misunderstanding — The Antardasha isn't something that 'changes every year.' Depending on the planetary combination, lengths range from a few months to several years.
Third, the most important thing is the order for reading the result. "Which planet's Dasha" alone tells you nothing. Check, in order: ① which Bhava it's in ② which Bhava it rules ③ its condition in the Rashi (dignity, combustion, retrograde) ④ its Nakshatra's ruling planet ⑤ its conjunctions and Drishti with other planets ⑥ its condition in D9 — only then does the grain of that period emerge.
This content is for reference and entertainment, based on traditional Chinese metaphysics and astrology.