Possble Calendar bug with daylight saving time

[Dan Howard]

So my cell phone rings this morning early and it's support calling to tell me that users are reporting strange errors in the application. When I have a look I find the problem is that we're comparing a date submitted via a web form to a date previously stored. The date submitted is in the format YYYYMMDDhhmm and it looks good though when I debug and look at the Calendar object created from this string it's an hour ahead!

Somehow on DST days the clock is being pushed up one more hour than it should. This is on a Windows machine in a DST time zone (for me Montreal Eastern Standard Time).

Below is a simple program to demonstrate this problem. It seems that the Calendar add method is the culprit. If I create a calendar by setting just the date and then 'adding' the minutes it bumps an hour up. If I just set each Calendar field it works fine.

Anyone want to confirm this? This is java 5_07 and java 6. TIA.

   
import java.util.Calendar;
import java.text.SimpleDateFormat;
 
/**
 * Demonstrates a possible bug in Calendar when adjusting a Calendar object by using the add method
 * to add minutes when DST is on.
 */
public class TestDSTBug {
 
    public static void main(String args[]) {
 
        // Calendar instance
        Calendar cal;
 
        // Date string to test. March 11, 2007 11:02 AM
        String dateString = "200703111102";
 
        cal = Calendar.getInstance();
 
        // Parse out the year, month and day
        int year, month, day;
        year = Integer.parseInt(dateString.substring(0, 4), 10);
        //in java months start at 0:
        month = Integer.parseInt(dateString.substring(4, 6), 10) - 1;
        day = Integer.parseInt(dateString.substring(6, 8), 10);
 
        System.out.println("Parsed out ymd: " + dateString + " " + year + " " + month + " " + day);
 
        // Set the calendar value
        cal.clear();
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.MONTH, month);
        cal.set(Calendar.DAY_OF_MONTH, day);
 
        // Simple format to display problem
        SimpleDateFormat sf = new SimpleDateFormat("yyyy/MM/dd HH:mm");
        System.out.println("Formatted before adding minutes : " + sf.format(cal.getTime()));
 

        // Calculate the minutes in the day from 1102 AM = 662 minutes from midnight
        int minutesInDay;
        String minutesString = dateString.substring(8);
        int hour = Integer.parseInt(minutesString.substring(0,2));
        int minute = Integer.parseInt(minutesString.substring(2));
 
        minutesInDay = hour * 60 + minute;
        System.out.println("Minutes in day:"  + minutesInDay);
 
        // Now let's add the minutes.
        cal.add(Calendar.MINUTE, minutesInDay);
        System.out.println("Formatted after add (note the hour is +1 !): " + sf.format(cal.getTime()));
 

        cal.clear();
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.MONTH, month);
        cal.set(Calendar.DAY_OF_MONTH, day);
        cal.set(Calendar.HOUR, hour);
        cal.set(Calendar.MINUTE, minute);
 
        System.out.println("Setting all the fields without using add and we see the correct date/time: " + sf.format(cal.getTime()));
 
    }
}

[Paul Clapham]

// Calculate the minutes in the day from 1102 AM = 662 minutes from midnight

This is incorrect on a day when DST starts. The answer in this case is 602 minutes because the second after 1:59:59 was 3:00:00. So if you are saying your calculations don't come out to 662 minutes, that isn't a bug in the Calendar class. It's the way it's supposed to be.

[Dan Howard]

I get it now. Now I know why this occured only on the DST day.

Thanks!