Calculate the number of days passed from a given year
Your code scales poorly if there are many years between STARTING_YEAR and localDate.

Worse, if localDate occurs before the STARTING_YEAR, then your code behaves as if STARTING_YEAR is the same year as localDate, which is weird behavior.

To calculate the number of days between two dates, use DAYS.between().

import java.time.LocalDate;
import static java.time.temporal.ChronoUnit.DAYS;

…    
long jdn = 1 + DAYS.between(LocalDate.ofYearDay(STARTING_YEAR, 1), localDate);


The 1 + is necessary because the convention when subtracting dates is inclusive-exclusive, whereas your code uses an inclusive-inclusive range.