今天在Java中的日期 - 我已经尝试了常规方法

Question

I need today's date - and zero anything else (" 05/06/08 00:00:00 ")

I've tried

Calendar calendar = Calendar.getInstance(); 
calendar.set(Calendar.HOUR, 0);        
Date date1 = calendar.getTime();                             
System.out.println(date1);

Run: (This is seriously messed up)

If the hour on the computer is < 12:00 at noon : Sun Mar 08 00:44:39 IST 2009

If the hour on the computer is > 12:00 at noon : Sun Mar 08 12:46:53 IST 2009

So I gave this up.

All the Date's setters are deprecated (except the epoch time) - so I don't want to use them either

The only thing I could think of is

Calendar calendar = Calendar.getInstance();     
SimpleDateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy");
String sDate = dateFormat.format(calendar.getTime());
Date today = dateFormat.parse(sDate);

But this is such a lame code I can't bring myself to write it.

Any other option?

Thanks!

Answer 1

I use this:

public static Date startOfDay(Date date) {
   Calendar dCal = Calendar.getInstance();
   dCal.setTime(date);
   dCal.set(Calendar.HOUR_OF_DAY, 0);
   dCal.set(Calendar.MINUTE, 0);
   dCal.set(Calendar.SECOND, 0);
   dCal.set(Calendar.MILLISECOND, 0);

   return dCal.getTime();
 }

Answer 2

My standard advice for Java date/time questions: don't use java.util.{Calendar,Date} . Use Joda Time . That way you can represent a date as a date (with no associated time zone), instead of a date/time. Or you could use a DateMidnight if that's what you want to represent. (Be careful of combinations of time zone and date where there is no midnight though...)

What do you need to use the Date with? If you can get away with changing to use Joda throughout, that's great. Otherwise, you can use Joda to do what you want and then convert to milliseconds (and then to java.util.Date ) when you really need to.

(Michael's solution when using Date / Calendar is fine if you really want to stick within a broken API... but I can't overstate how much better Joda is...)

Answer 3

You should use HOUR_OF_DAY instead of HOUR and combine it with MINUTE and SECOND also.

import java.util.Calendar;
import static java.util.Calendar.HOUR_OF_DAY;
import static java.util.Calendar.MINUTE;
import static java.util.Calendar.SECOND;
import static java.util.Calendar.MILLISECOND;

public class Today { 
    public static void main( String [] args ) { 
        Calendar cal = Calendar.getInstance();
        cal.set( HOUR_OF_DAY, 0 );
        cal.set( MINUTE, 0 );
        cal.set( SECOND, 0 );
        cal.set( MILLISECOND, 0 );
        System.out.println( cal.getTime() );
    }
}

The results you are getting are due to HOUR is used to AM/PM while HOUR_OF_DAY is 24 hrs.

HOUR_OF_DAY:

Field number for get and set indicating the hour of the day. HOUR_OF_DAY is used for the 24-hour clock. Eg, at 10:04:15.250 PM the HOUR_OF_DAY is 22.

HOUR:

Field number for get and set indicating the hour of the morning or afternoon. HOUR is used for the 12-hour clock (0 - 11). Noon and midnight are represented by 0, not by 12. Eg, at 10:04:15.250 PM the HOUR is 10.

Answer 4

The time component is not just hours (and Calendar.HOUR is, as you have noticed, AM/PM). You need to set all of the time fields to 0: HOUR_OF_DAY, MINUTE, SECOND, MILLISECOND.

Answer 5

请参阅Apache的commons-lang DateUtils.truncate（）

Answer 6

I know this is a very old question, no longer active, but it came to be on the top when I searched Google.

While all advise is very good, I can't believe no one simply answered:

Date date = new Date(System.currentTimeMillis());
System.out.println(date);

Which returns effectively, today's date.

Answer 7

...or you can do it the hacker way:

long MS_PER_DAY = 86400000L;
Date dateTime=new Date();
long offset = TimeZone.getDefault().getOffset(dateTime.getTime());
Date date= new Date(((dateTime.getTime()+offset)/MS_PER_DAY)*MS_PER_DAY-offset);

Answer 8

As mentioned above you should use

Calendar.HOUR_OF_DAY

As opposed to

Calendar.HOUR

Also you need to clear out the other fields ( Calendar.MINUTE , Calendar.SECOND , and Calendar.MILLISECOND ) by setting them to zero.

Sorry there's no easy way here. A pain, and that's why they're working on a new API for Java 7 I believe based on Joda Time.

Answer 9

Why the string manipulation?

Can you not just set the values you need on the Calendar object before converting to a Date using getTime()?

Answer 10

Another vote for JodaTime.

java.util.Date and Calendar are so bad they are broken. (And SimpleDateFormat is rubbish too!)

For what it's worth, Java 7 will include a new date time library based strongly around JodaTime.