Java: Get number of weeks between two dates

Below snippet return number of weeks between two dates.

private static long getFullWeeks(Calendar cal1, Calendar cal2) {

         Instant instant1 = Instant.ofEpochMilli(cal1.getTimeInMillis());

         Instant instant2 = Instant.ofEpochMilli(cal2.getTimeInMillis());

         LocalDateTime startDate = LocalDateTime.ofInstant(instant1, ZoneId.systemDefault());

         LocalDateTime endDate = LocalDateTime.ofInstant(instant2, ZoneId.systemDefault());

         return ChronoUnit.WEEKS.between(startDate, endDate);

}

App.java

package com.sample.app;

import static java.util.Calendar.DATE;
import static java.util.Calendar.HOUR;
import static java.util.Calendar.MILLISECOND;
import static java.util.Calendar.MINUTE;
import static java.util.Calendar.MONTH;
import static java.util.Calendar.SECOND;
import static java.util.Calendar.YEAR;

import java.text.DateFormatSymbols;
import java.time.Instant;
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.temporal.ChronoUnit;
import java.util.Calendar;

public class App {

 public static long getWeeks(Calendar cal1, Calendar cal2) {
  if (cal1.compareTo(cal2) <= 0) {
   return getFullWeeks(cal1, cal2);
  }

  return getFullWeeks(cal2, cal1);
 }

 private static long getFullWeeks(Calendar cal1, Calendar cal2) {

  Instant instant1 = Instant.ofEpochMilli(cal1.getTimeInMillis());
  Instant instant2 = Instant.ofEpochMilli(cal2.getTimeInMillis());

  LocalDateTime startDate = LocalDateTime.ofInstant(instant1, ZoneId.systemDefault());
  LocalDateTime endDate = LocalDateTime.ofInstant(instant2, ZoneId.systemDefault());

  return ChronoUnit.WEEKS.between(startDate, endDate);
 }

 private static String calToString(Calendar cal) {
  StringBuilder builder = new StringBuilder();
  builder.append(cal.get(YEAR)).append("-").append(getMonthFromInt(cal.get(MONTH))).append("-")
    .append(cal.get(DATE)).append(" ").append(cal.get(HOUR)).append(":").append(cal.get(MINUTE)).append(":")
    .append(cal.get(SECOND)).append(":").append(cal.get(MILLISECOND));

  return builder.toString();
 }

 private static String getMonthFromInt(int num) {

  if (num < 0 || num > 11) {
   throw new IllegalArgumentException("Months should be in range of 0 to 11");
  }
  DateFormatSymbols dfs = new DateFormatSymbols();
  String[] months = dfs.getMonths();
  return months[num];
 }

 public static void main(String args[]) {
  Calendar cal1 = Calendar.getInstance();
  Calendar cal2 = Calendar.getInstance();

  cal1.set(2019, 5, 22, 10, 45);
  cal2.set(2016, 5, 22, 10, 45);

  long noOfWeeks = getWeeks(cal1, cal2);

  System.out.println(
    "There are " + noOfWeeks + " weeks between " + calToString(cal1) + " and " + calToString(cal2));
 }
}

Output

There are 156 weeks between 2019-June-22 10:45:34:853 and 2016-June-22 10:45:34:863

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