What's the best way to manipulate Dates and Timestamps in Java?

Question

Every time I need to work with date and/or timstamps in Java I always feel like I'm doing something wrong and spend endless hours trying to find a better way of working with the APIs without having to code my own Date and Time utility classes. Here's a couple of annoying things I just ran into:

• 0-based months. I realize that best practice is to use Calendar.SEPTEMBER instead of 8, but it's annoying that 8 represents September and not August.

• Getting a date without a timestamp. I always need the utility that Zeros out the timestamp portion of the date.

• I know there's other issues I've had in the past, but can't recall. Feel free to add more in your responses.

So, my question is ... What third party APIs do you use to simplify Java's usage of Date and Time manipulation, if any? Any thoughts on using Joda? Anyone looked closer at JSR-310 Date and Time API?

Answer 1

java.time

Java 8 and later now includes the java.time framework. Inspired by Joda-Time, defined by JSR 310, extended by the ThreeTen-Extra project. See the Tutorial.

This framework supplants the old java.util.Date/.Calendar classes. Conversion methods let you convert back and forth to work with old code not yet updated for the java.time types.

The core classes are:

• Instant
  A moment on the timeline, always in UTC.
• ZoneId
  A time zone. The subclass ZoneOffset includes a constant for UTC.
• ZonedDateTime = Instant + ZoneId
  Represents a moment on the timeline adjusted into a specific time zone.

This framework solves the couple of problems you listed.

0-based months

Month numbers are 1-12 in java.time.

Even better, an Enum (Month) provides an object instance for each month of the year. So you need not depend on "magic" numbers in your code like 9 or 10.

if ( theMonth.equals ( Month.OCTOBER ) ) {  …

Furthermore, that enum includes some handy utility methods such as getting a month’s localized name.

If not yet familiar with Java enums, read the Tutorial and study up. They are surprisingly handy and powerful.

A date without a time

The LocalDate class represents a date-only value, without time-of-day, without time zone.

LocalDate localDate = LocalDate.parse( "2015-01-02" );

Note that determining a date requires a time zone. A new day dawns earlier in Paris than in Montréal where it is still ‘yesterday’. The ZoneId class represents a time zone.

LocalDate today = LocalDate.now( ZoneId.of( "America/Montreal" ) );

Similarly, there is a LocalTime class for a time-of-day not yet tied to a date or time zone.

---

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
  • See How to use ThreeTenABP….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Answer 2

This post has a good discussion on comparing the Java Date/Time API vs JODA.

I personally just use Gregorian Calendar and SimpleDateFormat any time I need to manipulate dates/times in Java. I've never really had any problems in using the Java API and find it quite easy to use, so have not really looked into any alternatives.

Answer 3

The Apache Commons Lang project has a DateUtils class that performs helpful Date operations.

I use DateUtils.truncate() a lot, which will "zero out" parts of the Date for you (helpful if you want your Date object to, say, represent a date and not include any time information). Each method works for both Date and Calendar objects too.

http://commons.apache.org/lang/

Answer 4

I've been using Joda exclusively for three years now and would definitely recommend it - it has the whole area covered with an interface that 'does what it says'.

Joda can look complex when you start, as eg it has concepts of periods, duration and intervals which look sort of similar, but you can start off simply by substituting org.joda.time.DateTime (or org.joda.time.DateMidnight) for java.util.Date in your code, and sticking with the many useful methods that those classes contain, before understanding the other areas.

Answer 5

Im using GregorianCalendar - always and everywhere. Simple java.util.Date is too complex, yeah.

So, my advice is - use GC, its simple

Answer 6

It's the same in javascript. Someone must have been smoking something when they think it's a good idea to let 2008 mean the year 2008, 31 to mean the 31st day in the month, and - this is the best part - 11 to mean the 12th month.

On the other hand, they got it right on two out of three.

Answer 7

The thing that always gets me with Java is the date time library. I've never used Joda, just briefly look at it, looks like its a pretty good implementation, and if I understand JSR-130 correctly its taking knowledge from Joda and eventually having it included in JavaSE.

Quite often for past projects I've wrapped the Java date time objects, which in itself was quite a task. Then used the wrappers for date manipulation.

Answer 8

Date APIs are very difficult to design, especially if they have to deal with localization. Try to roll your own and see, it's worth doing at least once. The fact that Joda was able to do such a good job is a real credit to its developers. To answer your question, I've heard nothing but good things about that library, though I have never played around with it myself.

Answer 9

A lot of programmers begin by using Date, which has numerous deprecated overloaded constructors (making it difficult to use), but once you figure out GregorianCalendar it becomes a little bit easier to manage. The example here is pretty helpful:

http://java.sun.com/j2se/1.4.2/docs/api/java/util/GregorianCalendar.html

Answer 10

It's really simple to write your own date API which sits on top of the raw Java classes, Date and Calendar. Basically both date and Calendar suffer from the fact that they are trying to cram two concepts into one class:

1. Date (i.e. Year-Month-Day)
2. Instant (i.e. currentTimeMillis)

When you understand this, it will just revolutionize how you handle date-like concepts in your code. Things will be simpler, clearer, better. In every sense!

For me, Joda is over-complicated, at least for the overwhelming majority of purposes and I particularly don't like the fact that they have gone against standard Java forms, one example being how they parse and format dates. Stephen Colebourne, the guy behind JODA, is the spec lead of JSR-310 and this suffers from the same problems imho (I've followed and contributed to the discussions for the last few years).

Do it yourself; it's easy. Just fill in the following classes: MyDate (wrapping year-month-day), Month (an enum), TimeOfDay (hour-min-sec-millis), DayOfWeek (enum), Instant (wrapping a long). Always consider time-zones when converting between Instants and Dates.

If this seems daunting, you can use Calendar and SimpleDateFormat under the hood. You'll do this in a day and never regret it.