I'm a beginner at Java I have a question about the new date

Think common sense.

In Date, the month is from 0 to 11.

19 is a year and eight months mark. (11 + 8)

For the same reason, 39 is 31, which is the remaining 8 days of the month.

So it's one more year, and the rest of August, and the date is 31 years old, so it's +1 month and it's September.