While referring to a particular course I came across the following code to retrieve the current day:
int julianStartDay = Time.getJulianDay(System.currentTimeMillis(), dayTime.gmtoff);
long dateTime;
dateTime = dayTime.setJulianDay(julianStartDay);
day = getReadableDateString(dateTime);
private String getReadableDateString(long time){
SimpleDateFormat shortenedDateFormat = new SimpleDateFormat("E MMM d");
return shortenedDateFormat.format(time);
}
My doubt is why are we using this julian method instead of just directly extracting the day from the Date class object which gives the current date and time.
Is your application related to the Eastern Orthodox religious calendar? As far as I'm aware, that's the only place the Julian calendar is still in use.
The Java Date and Calendar classes use the Gregorian calendar by default. (Calendar.getInstance methods return a GregorianCalendar object unless one of two specific locales is specified.) If your application needs to use the Julian calendar instead, that would explain this implementation.
Related
I'm trying to get the date from DateChooserCombo as follows
SimpleDateFormat sdf = new SimpleDateFormat("YYYY-MM-DD");
String date = sdf.format(dateChooser.getDate());
But the method getDate() gives me error (illegal forward reference). I have also tried with getSelectedDate() but it's the same. What can I do?
Anyway I'm using Apache Netbeans 12.1 and the date picker should be this one:
https://github.com/vadimig/jdatechooser
Thanks.
I downloaded the JDateChooser code from the link you provided in your question. There is no getDate() method in class datechooser.beans.DateChooserCombo. There is a getSelectedDate() method which returns an instance of class java.util.Calendar.
Also, according to the documentation for class java.text.SimpleDateFormat, the pattern YYYY-MM-DD is a valid pattern but I don't think it's the pattern that you want. D means day in year which means that 27th February is the 58th day of the year. You probably want d. Similarly, Y means Week year whereas you probably wanted y.
So, in order to get a string representation of the date that the user selected from the DateChooserCombo, you probably want the following code.
DateChooserCombo dcc = new DateChooserCombo(); // or however you create and configure it
Calendar cal = dcc.getSelectedDate();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
String date = sdf.format(cal.getTime());
By the way, it appears that JDateChooser development stopped seven years ago. Perhaps consider using JavaFX which has a DatePicker component which works with Java's date-time API.
This question already has answers here:
How to check if a date Object equals yesterday?
(9 answers)
Closed 8 years ago.
I have these codes to get the date today in my server
DefaultTableModel dnow = MyDB.DataTable("SELECT date_format(now(),'%m-%d-%Y')");
and these code for the formatting for the date.
SimpleDateFormat format = new SimpleDateFormat("MM-dd-yyyy");
now how can I get the date of yesterday?
should I just minus it with one?
An alternative approach is to do the math in SQL. The statement may vary depending on what database platform you're using.
DefaultTableModel dnow =
MyDB.DataTable("SELECT date_format(now() - INTERVAL 1 DAY,'%m-%d-%Y')");
No, build a Date (or better yet use joda's DateTime object) and then do your arithmetic. I will give you two solutions, one with Joda and the other without, starting with without:
Date d = format.parse(dnow.getDataVector().get(dnow.getTable().getSelectedRow()));
d.add(Calendar.DATE, -1);
Now, using joda:
DateTime d = new DateTime(format.parse(dnow.getDataVector().get(dnow.getTable().getSelectedRow()))).minusDays(1);
This should work perfect for you,
Calendar cal = Calendar.getInstance()
cal.add(Calendar.DATE, -1);
System.out.println("Yesterday's date = "+ cal.getTime());
This will simply subtract 1 day from the current calendar date, providing you yesterdays date
If you store your timestamps internally as POSIX times (milliseconds since MN Jan 1, 1970) then you can add or subtract a day to any time stamp as easily as:
Date today = new Date(); // today
Date tomorrow = new Date(today.getTime() + (24 * 60 * 60 * 1000)); // tomorrow
The huge bonus to POSIX time is that it is always in UTC. UTC is a global, "fixed" point of reference. Then if you need to need to have this date displayed for the user in any time zone, in any daylight savings zone, for any place that is accounted for in the Olson Time Zone database, you can simply create a Calendar:
GregorianCalendar gc = new GregorianCalendar(TimeZone.getTimeZone("America/Chicago"));
SimpleDateFormat sdf = new SimpleDateFormat();
sdf.setCalendar(gc);
sdf.applyPattern("MM-dd-yyyy");
// Now your formatter is set with the pattern and a time zone-aware Calendar.
// The world is at your command
String myOutput = sdf.format(tomorrow);
I highly recommend dealing with timestamps internally in your data model in some form of UTC representation. Doing date arithmetic with POSIX time (or Julian Day Numbers, or Modified Julian Day Numbers) is easy peasy, and the Java date/time API has enough capability to deal with most local time conversions without much fuss from you.
I need to generate a new Date object for credit card expiration date, I only have a month and a year, how can I generate a Date based on those two? I need the easiest way possible. I was reading some other answers on here, but they all seem too sophisticated.
You could use java.util.Calendar:
Calendar calendar = Calendar.getInstance();
calendar.clear();
calendar.set(Calendar.MONTH, month);
calendar.set(Calendar.YEAR, year);
Date date = calendar.getTime();
java.time
Using java.time framework built into Java 8
import java.time.YearMonth;
int year = 2015;
int month = 12;
YearMonth.of(year,month); // 2015-12
from String
YearMonth.parse("2015-12"); // 2015-12
with custom DateTimeFormatter
import java.time.format.DateTimeFormatter;
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MM yyyy");
YearMonth.parse("12 2015", formatter); // 2015-12
Conversions
To convert YearMonth to more standard date representation which is LocalDate.
LocalDate startMonth = date.atDay(1); //2015-12-01
LocalDate endMonth = date.atEndOfMonth(); //2015-12-31
Possibly a non-answer since you asked for a java.util.Date, but it seems like a good opportunity to point out that most work with dates and times and calendars in Java should probably be done with the Joda-Time library, in which case
new LocalDate(year, month, 1)
comes to mind.
Joda-Time has a number of other nice things regarding days of the month. For example if you wanted to know the first day of the current month, you can write
LocalDate firstOfThisMonth = new LocalDate().withDayOfMonth(1);
In your comment you ask about passing a string to the java.util.Date constructor, for example:
new Date("2012-09-19")
This version of the constructor is deprecated, so don't use it. You should create a date formatter and call parse. This is good advice because you will probably have year and month as integer values, and will need to make a good string, properly padded and delimited and all that, which is incredibly hard to get right in all cases. For that reason use the date formatter which knows how to take care of all that stuff perfectly.
Other earlier answers showed how to do this.
Like
SimpleDateFormat formatter = new SimpleDateFormat("yyyy/MM");
Date utilDate = formatter.parse(year + "/" + month);
Copied from Create a java.util.Date Object from a Year, Month, Day Forma
or maybe like
DateTime aDate = new DateTime(year, month, 1, 0, 0, 0);
Copied from What's the Right Way to Create a Date in Java?
The most common sense approach would be to use the Date("YYYY-MM-DD") constructor even though it is deprecated. This is the easiest way to create a date on the fly. Screw whoever decided to deprecate it. Long live Date("YYYY-MM-DD")!!!
Don’t use this answer. Use the answers by Przemek and Ray Toel. As Przemek says, prefer to use a YearMonth for representing year and month. As both say, if you must use a date, use LocalDate, it’s a date without time of day.
If you absolutely indispensably need an old-fashioned java.util.Date object for a legacy API that you cannot change, here’s one easy way to get one. It may not work as desired, it may not give you exactly the date that you need, it depends on your exact requirements.
YearMonth expiration = YearMonth.of(2021, 8); // or .of(2021, Month.AUGUST);
Date oldFashionedDateObject = Date.from(expiration
.atDay(1)
.atStartOfDay(ZoneId.systemDefault())
.toInstant());
System.out.println(oldFashionedDateObject);
On my computer this prints
Sun Aug 01 00:00:00 CEST 2021
What we got is the first of the month at midnight in my local time zone — more precisely, my JVM’s time zone setting. This is one good guess at what your legacy API expects, but it is also dangerous. The JVM’s time zone setting may be changed under our feet by other parts of the program or by other programs running in the same JVM. In other words, we cannot really be sure what we get.
The time zone issue gets even worse if the date is transmitted to a computer running a different time zone, like from client to server or vice versa, or to a database running its own time zone. There’s about 50 % risk that your Date will come through as a time in the previous month.
If you know the time zone required in the end, it will help to specify for example ZoneId.of("America/New_York") instead of the system default in the above snippet.
If your API is lenient and just needs some point within the correct month, you’ll be better off giving it the 2nd of the month UTC or the 3rd of the month in your own time zone. Here’s how to do the former:
Date oldFashionedDateObject = Date.from(expiration
.atDay(2)
.atStartOfDay(ZoneOffset.UTC)
.toInstant());
I know there are other similar questions to this, but I came up with my own way of getting the current time in a specific time zone, so I just wanted to confirm if it is correct or not, or there are gotchas I didn't take care of.
Calendar cal = Calendar.getInstance();
// Assuming we want to get the current time in GMT.
TimeZone tz = TimeZone.getTimeZone("GMT");
cal.setTimeInMillis(calendar.getTimeInMillis()
+ tz.getOffset(calendar.getTimeInMillis())
- TimeZone.getDefault().getOffset(calendar.getTimeInMillis()));
// Calendar should now be in GMT.
Is the above correct at all? I did my own test and it seemed to be working as expected, but just wanted to confirm it again with the experts in Stack Overflow.
If you simply do a Calendar.getInstance with the TimeZone argument, the calendar's internal state for the get() methods will return you the field with the time for that timezone. For example:
Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
// if i run this at 9 EST this will print 2
System.out.println(cal.get(Calendar.HOUR));
If you just need the local time for display purposes, you can set the TimeZone on your format object. For example:
SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss z");
sdf.setTimeZone(TimeZone.getTimeZone("GMT"));
System.out.println(sdf.format(new Date()));
Like Macarse said though, Joda time is where it's at if you need to do anything more complex. The Java date APIs are a nightmare.
I'd prefer using joda-time instead of that.
Check this link.
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!
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();
}
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...)
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. E.g., 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. E.g., at 10:04:15.250 PM the HOUR is 10.
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.
See Apache's commons-lang DateUtils.truncate()
java.time
The java.util Date-Time API and their formatting API, SimpleDateFormat are outdated and error-prone. It is recommended to stop using them completely and switch to the modern Date-Time API*.
Also, quoted below is a notice from the home page of Joda-Time:
Note that from Java SE 8 onwards, users are asked to migrate to java.time (JSR-310) - a core part of the JDK which replaces this project.
Solution using java.time, the modern Date-Time API:
The modern Date-Time API has many types which truly represent a date or time or date-time in a specific timezone. You can choose from the following options as per your specific requirement:
If you are looking for a type that represents a date without a timezone, you can use LocalDate.now. The good news is that its variant, LocalDate#now(ZoneId) returns the current date from the system clock in the specified time-zone.
If you are looking for an object that represents a date without a timezone, and with time units set to zero, you can call LocalDate#atStartOfDay on the object obtained with Option#1.
If you are looking for an Instant representing the Date-Time object obtained with Option#2, you can attach this object with ZoneId.of("Etc/UTC") using LocalDateTime#atZone to obtain a ZonedDateTime and convert the same into an Instant using ZonedDateTime#toInstant.
Demo:
import java.time.Instant;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
public class Main {
public static void main(String[] args) {
LocalDate todayInSystemTz = LocalDate.now();
System.out.println(todayInSystemTz);
LocalDate todayInIndia = LocalDate.now(ZoneId.of("Asia/Kolkata"));
System.out.println(todayInIndia);
LocalDateTime todayInSystemTzWithZeroTimeUnits = todayInSystemTz.atStartOfDay();
System.out.println(todayInSystemTzWithZeroTimeUnits);
ZonedDateTime todayInUtcWithZeroTimeUnits = todayInSystemTzWithZeroTimeUnits.atZone(ZoneId.of("Etc/UTC"));
System.out.println(todayInUtcWithZeroTimeUnits);
Instant instant = todayInUtcWithZeroTimeUnits.toInstant();
System.out.println(instant);
// Can I represent the obtained Instant in India?
System.out.println(instant.atZone(ZoneId.of("Asia/Kolkata")));
// Can I represent the obtained Instant in New York?
System.out.println(instant.atZone(ZoneId.of("America/New_York")));
}
}
Output:
2021-06-20
2021-06-20
2021-06-20T00:00
2021-06-20T00:00Z[Etc/UTC]
2021-06-20T00:00:00Z
2021-06-20T05:30+05:30[Asia/Kolkata]
2021-06-19T20:00-04:00[America/New_York]
ONLINE DEMO
The Z in the output is the timezone designator for zero-timezone offset. It stands for Zulu and specifies the Etc/UTC timezone (which has the timezone offset of +00:00 hours).
For any reason, if you need to convert this object of Instant to an object of java.util.Date**, you can do so as follows:
Date date = Date.from(instant);
Learn more about the modern Date-Time API from Trail: Date Time.
* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.
**
A java.util.Date object simply represents the number of milliseconds since the standard base time known as "the epoch", namely January 1, 1970, 00:00:00 GMT (or UTC). Since it does not hold any timezone information, its toString function applies the JVM's timezone to return a String in the format, EEE MMM dd HH:mm:ss zzz yyyy, derived from this milliseconds value. To get the String representation of the java.util.Date object in a different format and timezone, you need to use SimpleDateFormat with the desired format and the applicable timezone e.g.
Date date = new Date();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSXXX", Locale.ENGLISH);
sdf.setTimeZone(TimeZone.getTimeZone("America/New_York"));
String strDateNewYork = sdf.format(date);
sdf.setTimeZone(TimeZone.getTimeZone("Etc/UTC"));
String strDateUtc = sdf.format(date);
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.
...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);
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.
Why the string manipulation?
Can you not just set the values you need on the Calendar object before converting to a Date using getTime()?
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.