Date对象和Calendar之间的日期计算不同

Question

I am writing a code to add amount of days to current date. I have 2 solutions:

SOLUTION 1

Date current = new Date();

// Add 100 days
Date d = new Date ( current.getTime() + 100 * 24 * 60 * 60 * 1000 );

System.out.println(d);

SOLUTION 2

Date current = new Date();

GregorianCalendar c = new GregorianCalendar();
c.setTime(current);
c.add(Calendar.DATE, 100);

System.out.println(c.getTime());

RESULT

SOLUTION 1: Sat Apr 05 14:56:33 CDT 2014

SOLUTION 2: Sat Apr 05 13:56:33 CDT 2014

The result of solution_2 is LESS than result of solution_1 ONE hour.

Anyone has any ideas? Thanks!

Answer 1

Solution 2 takes daylight savings into account, whereas solution 1 doesn't. Hence the difference.

On days when there is a transition from summer to winter time, or vice versa, the day isn't 24 hours long as the first method assumes.

Answer 2

Daylight Saving Time

Six months from now will cross the beginning of Daylight Saving Time in Lincoln, Nebraska US.

Sunday, March 9, 2014 at 2:00:00 AM clocks are turned forward 1 hour to
Sunday, March 9, 2014 at 3:00:00 AM local daylight time instead

http://www.timeanddate.com/worldclock/clockchange.html?n=132&year=2014

Daylight Saving Time (DST) is one very good reason to avoid doing your own date-time calculations. Use a competent library instead.

Joda-Time

Neither Solution 1 nor Solution 2 are good. The java.util.Date/Calendar classes are notoriously bad and should be avoided. They are being supplanted in Java 8 by the new java.time.* classes , whose design was inspired by the Joda-Time library.

My example code below uses Joda-Time 2.3 running in Java 7.

Time Zone

Your code fails to address the issue of time zones. Specifying a time zone is a better practice than depending on the default time zone. My example code is using a time zone for Lincoln, NE, US ("America/Indiana/Knox").

Example Code

// © 2013 Basil Bourque. This source code may be used freely forever by anyone taking full responsibility for doing so.
// import org.joda.time.*;
// import org.joda.time.format.*;

// Specify a time zone rather than rely on defaults.
DateTimeZone timeZone = DateTimeZone.forID( "America/Indiana/Knox" );

DateTime dateTime = new DateTime( timeZone );
DateTime dateTimeLater = dateTime.plusDays( 100 );

// If you are focusing on the day itself rather than a particular time of day, then use first moment of the day.
DateTime dateTimeStartOfDay = new DateTime( timeZone ).withTimeAtStartOfDay();
DateTime dateTimeLaterStartOfDay = dateTimeStartOfDay.plusDays( 100 ).withTimeAtStartOfDay();

Dump to console…

System.out.println( "dateTime: " + dateTime );
System.out.println( "dateTimeLater: " + dateTimeLater );
System.out.println( "dateTimeStartOfDay: " + dateTimeStartOfDay );
System.out.println( "dateTimeLaterStartOfDay: " + dateTimeLaterStartOfDay );
System.out.println( "dateTimeLaterStartOfDay in UTC: " + dateTimeLaterStartOfDay.toDateTime( DateTimeZone.UTC ) );

When run…

dateTime: 2013-12-26T23:12:33.527-06:00
dateTimeLater: 2014-04-05T23:12:33.527-05:00
dateTimeStartOfDay: 2013-12-26T00:00:00.000-06:00
dateTimeLaterStartOfDay: 2014-04-05T00:00:00.000-05:00
dateTimeLaterStartOfDay in UTC: 2014-04-05T05:00:00.000Z

Convert To juDate

If you need to convert to a java.util.Date for use with other code, call the toDate method.

java.util.Date date = dateTime.toDate();