Calculate the number of weeks "between" two dates, inclusive.

deltaWeekCalculator.java

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.ZoneId;
import java.time.temporal.TemporalAdjusters;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

public class Main {
    public static void main(String[] args) throws ParseException {
        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
        Date start = dateFormat.parse("2019-01-01"),
                end = dateFormat.parse("2019-01-21");

        int weeks1 = option1(start, end);
        int weeks2 = option2(start, end);
        int weeks3 = option3(start, end);

        System.out.println("Weeks:\n\tOption 1: " + weeks1 + "\n\tOption 2: " + weeks2 + "\n\tOption 3: " + weeks3);
    }

    public static int option1(Date start, Date end) {
        Calendar cal = new GregorianCalendar();
        cal.setTime(start);

        int weeks = 0;
        while (cal.getTime().before(end)) {
            cal.add(Calendar.WEEK_OF_YEAR, 1);
            weeks++;
        }
        return weeks;
    }

    public static int option2(Date start, Date end) {
        Calendar a = new GregorianCalendar();
        Calendar b = new GregorianCalendar();

        a.setTime(start);
        b.setTime(end);

        return b.get(Calendar.WEEK_OF_YEAR) - a.get(Calendar.WEEK_OF_YEAR);
    }

    public static int option3(Date start, Date end) {
        Date firstOfWeek = Date.from(start.toInstant().atZone(ZoneId.of("America/New_York")).toLocalDate().with(
                TemporalAdjusters.previousOrSame(DayOfWeek.SUNDAY)
        ).atStartOfDay(ZoneId.systemDefault()).toInstant());

        Date lastOfWeek = Date.from(end.toInstant().atZone(ZoneId.of("America/New_York")).toLocalDate().with(
                TemporalAdjusters.nextOrSame(DayOfWeek.SATURDAY)
        ).atStartOfDay(ZoneId.systemDefault()).toInstant());

        return option1(firstOfWeek, lastOfWeek);
    }
}