Calendar in Java… I get the same result for two different values

Question

    Calendar cal = Calendar.getInstance();
    long now = cal.getTimeInMillis();
    int year = 2014;
    int month = 3;
    int date = 8;
    cal.set(year, month - 1, date);
    long dday = cal.getTimeInMillis();
    long count = (dday - now) / (1000 * 24 * 60 * 60);
    System.out.println((dday - now));
    System.out.println(count);

This code figures out how many days between now and a date. However, I get the same result for Mar. 8 and Mar. 9. Please help! Thank you in advance.

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More detailed source code example, by Basil Bourque.

Calendar cal = Calendar.getInstance();
long nowInMillis = cal.getTimeInMillis();

cal.set( 2014, Calendar.MARCH, 8 );  // March 8, 2014   // year, month, day
long march_8_2014_inMillis = cal.getTimeInMillis();

cal.set( 2014,  Calendar.MARCH, 9 );  // March 9, 2014   // year, month, day
long march_9_2014_inMillis = cal.getTimeInMillis();

long daysTil8th = ( march_8_2014_inMillis - nowInMillis ) / ( 1000 * 24 * 60 * 60 );
long daysTil9th = ( march_9_2014_inMillis - nowInMillis ) / ( 1000 * 24 * 60 * 60 );

System.out.println( "( march_8_2014_inMillis - nowInMillis ) in artificial days: " + daysTil8th + ", in milliseconds: " + ( march_8_2014_inMillis - nowInMillis ) );
System.out.println( "( march_9_2014_inMillis - nowInMillis ) in artificial days: " + daysTil9th + ", in milliseconds: " + ( march_9_2014_inMillis - nowInMillis ) );

System.out.println( "( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ): " + ( ( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ) ) ) ;
System.out.println( "( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ) / ( 1000 * 60 * 60 ): " + ( ( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ) ) / ( 1000 * 60 * 60 ) ) ;

When run in Seattle, Java 7, Mac OS X 10.8.5 at 2013-11-20T17:35:01.119-08:00 (Pacific Standard Time)…

( march_8_2014_inMillis - nowInMillis ) in artificial days: 108, in milliseconds: 9331200000
( march_9_2014_inMillis - nowInMillis ) in artificial days: 108, in milliseconds: 9414000000
( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ): 82800000
( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ) / ( 1000 * 60 * 60 ): 23

NOTE: For my time zone, DST begins 2:00 AM on Sunday, March 9, 2014.

QUESTION: Why 108 in both lines of output above? The answer is DST, but how exactly?

Answer 1

3月9日是DST将于2014年转换的日子。因此，计算结果是正确的，3月9日的差异是108天23小时，即108天。

Answer 2

As Jim Garrison said in his correct answer , your integer-based math (1000 * 24 * 60 * 60) assumes 24-hours in a day. Not true for those of us in a time zone with the irrational practice of Daylight Saving Time (DST). Between the 8th and 9th in United States' time zones is 23 hours, not 24.

Lessons learned:

• Never do your own Date and Time math.
• Never use java.util.Date & .Calendar bundled in Java.
• Always use a decent library such as Joda-Time or the new JSR 310 in Java 8.

Here's the same problem solved in Java 7 with Joda-Time 2.3, and correct output.

// © 2013 Basil Bourque. This source code may be used freely forever by anyone taking full responsibility for doing so.

// Using US west coast time zone because March 9, 2014 at 02:00 is the start of DST Daylight Saving Time.
org.joda.time.DateTimeZone californiaTimeZone = org.joda.time.DateTimeZone.forID("America/Los_Angeles");

org.joda.time.DateTime now = new org.joda.time.DateTime(californiaTimeZone);

// Note the arbitrary choice of times (14 & 21 hours) assigned to new DateTimes.
org.joda.time.DateTime march_8_2014 = new org.joda.time.DateTime( 2014, 3, 8, 14, 0);
org.joda.time.DateTime march_9_2014 = new org.joda.time.DateTime( 2014, 3, 9, 21, 0);

int daysUntil8th = org.joda.time.Days.daysBetween( now , march_8_2014 ).getDays();
int daysUntil9th = org.joda.time.Days.daysBetween( now , march_9_2014 ).getDays();

// If you want whole days, call 'withTimeAtStartOfDate()' method.
// Don't get clever with trying to set a midnight time. Not every day in every time zone has a midnight.
int daysUntil8thStarts = org.joda.time.Days.daysBetween( now , march_8_2014.withTimeAtStartOfDay() ).getDays();
int daysUntil9thStarts = org.joda.time.Days.daysBetween( now , march_9_2014.withTimeAtStartOfDay() ).getDays();

System.out.println("Now in 'America/Los_Angeles' (Pacific Standard Time): " + now );

System.out.println("Days until " +  march_8_2014 + ": " + daysUntil8th );
System.out.println("Days until " + march_9_2014 + ": " + daysUntil9th );

System.out.println("Days until March 8, 2014 starts: " + daysUntil8thStarts );
System.out.println("Days until March 9, 2014 starts: " + daysUntil9thStarts );

When run…

Now in 'America/Los_Angeles' (Pacific Standard Time): 2013-11-20T18:25:06.424-08:00
Days until 2014-03-08T14:00:00.000-08:00: 107
Days until 2014-03-09T21:00:00.000-07:00: 109
Days until March 8, 2014 starts: 107
Days until March 9, 2014 starts: 108

NOTE: Time of day affects the count of days. Notice 107 & 109 versus 107 & 108, a two day difference versus one day difference.