Thursday, July 12, 2007

Java - calculate the difference between two dates

Finding the difference between two dates isn't as straightforward as subtracting the two dates and dividing the result by (24 * 60 * 60 * 1000). Infact, its erroneous! 

Going the 'milliseconds way' will lead to rounding off errors and they become most evident once you have a little thing like "Daylight Savings Time" come into the picture.

The Correct Way:

/** Using Calendar - THE CORRECT WAY**/
//assert: startDate must be before endDate
public static long daysBetween(Calendar startDate, Calendar endDate) {
  Calendar date = (Calendar) startDate.clone();
  long daysBetween = 0;
  while (date.before(endDate)) {
    date.add(Calendar.DAY_OF_MONTH, 1);
    daysBetween++;
  }
  return daysBetween;
}
}

or more efficiently, (thanks Mauro), if you're using the Gregorian Calendar:
/** Using Calendar - THE CORRECT (& Faster) WAY**/
/****Needs testing ...... Anyone?****/ 
//assert: startDate must be before endDate
public static long daysBetween(final Calendar startDate, final Calendar endDate) {
 int MILLIS_IN_DAY = 1000 * 60 * 60 * 24;
 long endInstant = endDate.getTimeInMillis();
 int presumedDays = (int) ((endInstant - startDate.getTimeInMillis()) / MILLIS_IN_DAY);
 Calendar cursor = (Calendar) startDate.clone();
 cursor.add(Calendar.DAY_OF_YEAR, presumedDays);
 long instant = cursor.getTimeInMillis();
 if (instant == endInstant)
  return presumedDays;
 final int step = instant < endInstant ? 1 : -1;
 do {
  cursor.add(Calendar.DAY_OF_MONTH, step);
  presumedDays += step;
 } while (cursor.getTimeInMillis() != endInstant);
 return presumedDays;
}



The Nuances:

Lets take, for example, the difference between the two dates
03/24/2007 and 03/25/2007 should be 1 day; 

However, using the millisecond route, you'll get 0 days, if you run this in the UK!


import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;

public class DateTest {

public class DateTest {

static SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy");

public static void main(String[] args) {

  TimeZone.setDefault(TimeZone.getTimeZone("Europe/London"));

  //diff between these 2 dates should be 1
  Date d1 = new Date("01/01/2007 12:00:00");
  Date d2 = new Date("01/02/2007 12:00:00");

  //diff between these 2 dates should be 1
  Date d3 = new Date("03/24/2007 12:00:00");
  Date d4 = new Date("03/25/2007 12:00:00");

  Calendar cal1 = Calendar.getInstance();cal1.setTime(d1);
  Calendar cal2 = Calendar.getInstance();cal2.setTime(d2);
  Calendar cal3 = Calendar.getInstance();cal3.setTime(d3);
  Calendar cal4 = Calendar.getInstance();cal4.setTime(d4);

  printOutput("Manual   ", d1, d2, calculateDays(d1, d2));
  printOutput("Calendar ", d1, d2, daysBetween(cal1, cal2));
  System.out.println("---");
  printOutput("Manual   ", d3, d4, calculateDays(d3, d4));
  printOutput("Calendar ", d3, d4, daysBetween(cal3, cal4));
}


private static void printOutput(String type, Date d1, Date d2, long result) {
  System.out.println(type+ "- Days between: " + sdf.format(d1)
                    + " and " + sdf.format(d2) + " is: " + result);
}

/** Manual Method - YIELDS INCORRECT RESULTS - DO NOT USE**/
/* This method is used to find the no of days between the given dates */
public static long calculateDays(Date dateEarly, Date dateLater) {
  return (dateLater.getTime() - dateEarly.getTime()) / (24 * 60 * 60 * 1000);
}

/** Using Calendar - THE CORRECT WAY**/
public static long daysBetween(Calendar startDate, Calendar endDate) {
  Calendar date = (Calendar) startDate.clone();
  long daysBetween = 0;
  while (date.before(endDate)) {
    date.add(Calendar.DAY_OF_MONTH, 1);
    daysBetween++;
  }
  return daysBetween;
}
}

OUTPUT:
Manual - Days between: 01-Jan-2007 and 02-Jan-2007 is: 1
Calendar - Days between: 01-Jan-2007 and 02-Jan-2007 is: 1
---
Manual - Days between: 24-Mar-2007 and 25-Mar-2007 is: 0
Calendar - Days between: 24-Mar-2007 and 25-Mar-2007 is: 1