Java Date Calendar DateFormat Details

From https://www.ntu.edu.sg/home/ehchua/programming/java/DateTimeCalendar.html

Date and Time - Creation, Operation and Formatting

Introduction

There are many Java classes available for date/time and it becomes pretty confusing, in particular, the Date and Calendar clases. The documentation is not very clear, and I have to look into the source codes to understand the salient features.

java.util.Date and java.text.SimpleDateFormat

Date is sufficient if you need only a current timestamp in your application, and you do not need to operate on dates, e.g., one-week later. You can further use SimpleDateFormat to control the date/time display format.

The class java.util.Date represents a specific instance in time, with millisecond precision. It has two constructors:

Date(): Allocate a Date instance with the current time.
Date(long millisSinceEpoch): Allocate a Date instance with the given time.

From the source code, the no-arg constructor invokes System.currentTimeMillis() to get the milliseconds since January 1, 1970, 00:00:00 GMT (known as "epoch") and stores in a private variable fastTime. Take note that epoch is absolute and does not depends on local time zones.

Most of the methods in Date are deprecated (for lack of internationalization support), except:

long getTime(): returns the number of milliseconds since epoch.
String toString(): returns a date/time string in local time-zone using the default locale in the format: dow mon dd hh:mm:ss zzz yyyy, where dowis the day of week (Sun, ..., Sat), mon is the month (Jan, ..., Dec), zzz is the time zone. Take note that although Date is represented based on the absolute epoch, the toString() displays the local time, according to the default time zone.

The Date's toString() method has a fixed date/time display format. You can use SimpleDateFormat to control the display format.

EXAMPLE:

import java.text.SimpleDateFormat;

import java.util.Date;

public class DateTest {

   public static void main(String[] args) {

      Date now = new Date();

      System.out.println("toString(): " + now);  // dow mon dd hh:mm:ss zzz yyyy

      // SimpleDateFormat can be used to control the date/time display format:

      //   E (day of week): 3E or fewer (in text xxx), >3E (in full text)

      //   M (month): M (in number), MM (in number with leading zero)

      //              3M: (in text xxx), >3M: (in full text full)

      //   h (hour): h, hh (with leading zero)

      //   m (minute)

      //   s (second)

      //   a (AM/PM)

      //   H (hour in 0 to 23)

      //   z (time zone)

      SimpleDateFormat dateFormatter = new SimpleDateFormat("E, y-M-d 'at' h:m:s a z");

      System.out.println("Format 1:   " + dateFormatter.format(now));

      dateFormatter = new SimpleDateFormat("E yyyy.MM.dd 'at' hh:mm:ss a zzz");

      System.out.println("Format 2:   " + dateFormatter.format(now));

      dateFormatter = new SimpleDateFormat("EEEE, MMMM d, yyyy");

      System.out.println("Format 3:   " + dateFormatter.format(now));

   }

}

toString(): Sat Sep 25 21:27:01 SGT 2010

Format 1:   Sat, 10-9-25 at 9:27:1 PM SGT

Format 2:   Sat 2010.09.25 at 09:27:01 PM SGT

Format 3:   Saturday, September 25, 2010

Summary:

Date class is sufficient if you just need a simple timestamp.
You could use SimpleDateFormat to control the date/time display format.

Use java.util.Calendar class if you need to extract year, month, day, hour, minute, and second, or manipulating these field (e.g., 7 days later, 3 weeks earlier).

Use java.text.DateFormat to format a Date (form Date to text) and parse a date string (from text to Date). SimpleDateForamt is a subclass of DateFormat.

Date is legacy class, which does not support internationalization. Calendar and DateFormat support locale (you need to consider locale only if you program is to be run in many countries concurrently).

java.util.Calendar & java.util.GregorianCalendar

The Calendar class provides support for:

maintaining a set of calendar fields such as YEAR, MONTH, DAY_OF_MONTH, HOUR, MINUTE, SECOND, MILLISECOND; and
manipulating these calendar fields, such as getting the date of the previous week, roll forward by 3 days.

Calendar provides internationalization support.

Calendar is a abstract class, and you cannot use the constructor to create an instance. Instead, you use the static method Calendar.getInstance() to instantiate an implementation sub-class.

Calendar.getInstance(): return a Calendar instance based on the current time in the default time zone with the default locale.
Calendar.getInstance(TimeZone zone)
Calendar.getInstance(Locale aLocale)
Calendar.getInstance(TimeZone zone, Locale aLocale)

Looking into the source code reveals that: getInstance() returns a GregorianCalendar instance for all locales, (except BuddhistCalendar for Thai ("th_TH") and JapaneseImperialCalendar for Japanese ("ja_JP")).

The most important method in Calendar is get(int calendarField), which produces an int. The calendarField are defined as static constant and includes:

get(Calendar.DAY_OF_WEEK): returns 1 (Calendar.SUNDAY) to 7 (Calendar.SATURDAY).
get(Calendar.YEAR): year
get(Calendar.MONTH): returns 0 (Calendar.JANUARY) to 11 (Calendar.DECEMBER).
get(Calendar.DAY_OF_MONTH), get(Calendar.DATE): 1 to 31
get(Calendar.HOUR_OF_DAY): 0 to 23
get(Calendar.MINUTE): 0 to 59
get(Calendar.SECOND): 0 to 59
get(Calendar.MILLISECOND): 0 to 999
get(Calendar.HOUR): 0 to 11, to be used together with Calendar.AM_PM.
get(Calendar.AM_PM): returns 0 (Calendar.AM) or 1 (Calendar.PM).
get(Calendar.DAY_OF_WEEK_IN_MONTH): DAY_OF_MONTH 1 through 7 always correspond to DAY_OF_WEEK_IN_MONTH 1; 8 through 14 correspond to DAY_OF_WEEK_IN_MONTH 2, and so on.
get(Calendar.DAY_OF_YEAR): 1 to 366
get(Calendar.ZONE_OFFSET): GMT offset value of the time zone.
get(Calendar.ERA): Indicate AD (GregorianCalendar.AD), BC (GregorianCalendar.BC).

A date in Calendar can be represented as:

YEAR + MONTH + DAY_OF_MONTH

YEAR + MONTH + WEEK_OF_MONTH + DAY_OF_WEEK

YEAR + MONTH + DAY_OF_WEEK_IN_MONTH + DAY_OF_WEEK

YEAR + DAY_OF_YEAR

YEAR + DAY_OF_WEEK + WEEK_OF_YEAR

A time in Calendar can be represented as:

HOUR_OF_DAY

AM_PM + HOUR

Example:

// Get the year, month, day, hour, minute, second

import java.util.Calendar;

public class GetYMDHMS {

   public static void main(String[] args) {

      Calendar cal = Calendar.getInstance();

      // You cannot use Date class to extract individual Date fields

      int year = cal.get(Calendar.YEAR);

      int month = cal.get(Calendar.MONTH);      // 0 to 11

      int day = cal.get(Calendar.DAY_OF_MONTH);

      int hour = cal.get(Calendar.HOUR_OF_DAY);

      int minute = cal.get(Calendar.MINUTE);

      int second = cal.get(Calendar.SECOND);

      System.out.printf("Now is %4d/%02d/%02d %02d:%02d:%02d\n",  // Pad with zero

          year, month+1, day, hour, minute, second);

   }

}

Calendar has these setters and operations:

void set(int calendarField, int value)
void set(int year, int month, int date)
void set(int year, int month, int date, int hour, int minute, int second)
void add(int field, int amount): Adds or subtracts the specified amount of time to the given calendar field, based on the calendar's rules.
void roll(int calendarField, boolean up): Adds or subtracts (up/down) a single unit of time on the given time field without changing larger fields.
void roll(int calendarField, int amount): Adds the specified (signed) amount to the specified calendar field without changing larger fields.

Other frequently-used methods are:

Date getTime(): return a Date object based on this Calendar's value.
void setTime(Date date)
long getTimeInMills(): Returns this Calendar's time value in milliseconds.
void setTimeInMillis(long millis)
void setTimeZone(TimeZone value)

Conversion between Calendar and Date

You can use getTime() and setTime() to convert between Calendar and Date.

Date getTime(): Returns a Date object representing this Calendar's time value

void setTime(Date aDate): Sets this Calendar's time with the given Date instance

EXAMPLE:

import java.util.Date;

import java.util.Calendar;

import java.text.SimpleDateFormat;

public class CalendarTest {

   public static void main(String[] args) {

      Calendar cal = Calendar.getInstance();   // GregorianCalendar

      System.out.println("Calendar's toString() is : " + cal + "\n");

      System.out.println("Time zone is: " + cal.getTimeZone() + "\n");

      // An Easier way to print the timestamp by getting a Date instance

      Date date = cal.getTime();

      System.out.println("Current date and time in Date's toString() is : " + date + "\n");

      // Print Calendar's field

      System.out.println("Year  : " + cal.get(Calendar.YEAR));

      System.out.println("Month : " + cal.get(Calendar.MONTH));

      System.out.println("Day of Month : " + cal.get(Calendar.DAY_OF_MONTH));

      System.out.println("Day of Week  : " + cal.get(Calendar.DAY_OF_WEEK));

      System.out.println("Day of Year  : " + cal.get(Calendar.DAY_OF_YEAR));

      System.out.println("Week of Year : " + cal.get(Calendar.WEEK_OF_YEAR));

      System.out.println("Week of Month : " + cal.get(Calendar.WEEK_OF_MONTH));

      System.out.println("Day of the Week in Month : " + cal.get(Calendar.DAY_OF_WEEK_IN_MONTH));

      System.out.println("Hour  : " + cal.get(Calendar.HOUR));

      System.out.println("AM PM : " + cal.get(Calendar.AM_PM));

      System.out.println("Hour of the Day : " + cal.get(Calendar.HOUR_OF_DAY));

      System.out.println("Minute : " + cal.get(Calendar.MINUTE));

      System.out.println("Second : " + cal.get(Calendar.SECOND));

      System.out.println();

      // Manipulating Dates

      Calendar calTemp;

      calTemp = (Calendar) cal.clone();

      calTemp.add(Calendar.DAY_OF_YEAR, -365);

      System.out.println("365 days ago, it was: " + calTemp.getTime());

      calTemp = (Calendar) cal.clone();

      calTemp.add(Calendar.HOUR_OF_DAY, 11);

      System.out.println("After 11 hours, it will be: " + calTemp.getTime());

      // Roll

      calTemp = (Calendar) cal.clone();

      calTemp.roll(Calendar.HOUR_OF_DAY, 11);

      System.out.println("Roll 11 hours, it will be: " + calTemp.getTime());

      System.out.println();

   }

}

java.util.GregorianCalendar

The calendar that we use today, called Gregorian calendar, came into effect in October 15, 1582 in some countries and later in other countries. It replaces the Julian calendar. 10 days were removed from the calendar, i.e., October 4, 1582 (Julian) was followed by October 15, 1582 (Gregorian). The only difference between the Gregorian and the Julian calendar is the "leap-year rule". In Julian calendar, every four years is a leap year. In Gregorian calendar, a leap year is a year that is divisible by 4 but not divisible by 100, or it is divisible by 400, i.e., the Gregorian calendar omits century years which are not divisible by 400 (removing 3 leap years (or 3 days) for every 400 years). Furthermore, Julian calendar considers the first day of the year as march 25th, instead of January 1st.

java.util.Calendar is an abstract class. Calendar.getInstance() returns an implementation class java.util.GregorianCalendar (except locales of "th" and "jp"). In Java, this GregorianCalendar handles both the Gregorian calendar as well as the Julian calendar, including the cut over.

GregorianCalendar has the following constructors:

GregorianCalendar(): using the current time, with the default time zone and locale.
GregorianCalendar(int year, int month, int dayOfMonth): with the default time zone and locale.
GregorianCalendar(int year, int month, int dayOfMonth, int hourOfDay, int minute, int second)
GregorianCalendar(TimeZone zone, Locale aLocale): using current time.
GregorianCalendar(TimeZone zone)
GregorianCalendar(Locale aLocale)

For example,

Calendar cal1 = new GregorianCalendar();  // allocate an instance and upcast to Calendar

Calendar cal2 = new GregorianCalendar(2010, 9, 26);  // allocate with the specified date

cal2.get(Calendar.DAY_OF_WEEK);                      // 1 (Sunday) to 7 (Saturday)

java.text.DateFormat & java.text.SimpleDateFormat

Read Java Tutorial, Internationalization ⇒ Formatting ⇒ Dates and Times at http://java.sun.com/docs/books/tutorial/i18n/format/dateintro.html.

java.text.DateFormat is an abstract class for formats (from date to text) and parses (from text to date) date/time in a text-language-independent manner.SimpleDateFormat is an implementation subclass. Date formatter operates on Date object.

To use the date formatter, first create a DateFormat object for the desired date/time format, and then use the format() method to produce a date/time string.

To use the DateFormat, use one of these static factory method to create an instance:

DateFormat.getDateTimeInstance(): use the default style and locale to format date and time.
DateFormat.getDateTimeInstance(int dateStyle, int timeStyle, Locale aLocale): style includes DateFormat.FULL, LONG, MEDIUM, and SHORT.
DateFormat.getInstance(): uses SHORT style for date and time.
DateFormat.getDateInstance(), DateFormat.getDateInstance(int style, Locale aLocale): date only.
DateFormat.getTimeInstance(), DateFormat.getTimeInstance(int style, Locale aLocale): time only.

To parse a text string int Date, use:

DateFormat formatter = ....

Date myDate = formatter.parse(myString);

EXAMPLE:

import java.util.Date;

import java.text.DateFormat;

import java.text.SimpleDateFormat;

import java.util.Locale;

public class DateFormatTest {

   public static void main(String[] args) {

      Date now = new Date();

      // Use Date.toString()

      System.out.println(now);

      // Use DateFormat

      DateFormat formatter = DateFormat.getInstance(); // Date and time

      String dateStr = formatter.format(now);

      System.out.println(dateStr);

      formatter = DateFormat.getTimeInstance();        // time only

      System.out.println(formatter.format(now));

      // Use locale

      formatter = DateFormat.getDateTimeInstance(DateFormat.FULL, DateFormat.FULL, Locale.FRANCE);

      System.out.println(formatter.format(now));

      // Use SimpleDateFormat

      SimpleDateFormat simpleFormatter = new SimpleDateFormat("E yyyy.MM.dd 'at' hh:mm:ss a zzz");

      System.out.println(simpleFormatter.format(now));

   }

}

Sun May 16 19:38:41 SGT 2010

5/16/10 7:38 PM

7:38:41 PM

dimanche 16 mai 2010 19 h 38 SGT

Sun 2010.05.16 at 07:38:41 PM SGT

Measuring Time

Many applications (such as games and animation) require good timing control.

Java provides these static methods in System class:

System.currentTimeMillis(): Returns the current time in milliseconds since January 1, 1970 00:00:00 GMT (known as "epoch"), in long.

// Measuring elapsed time

long startTime = System.currentTimeMillis();

// The code being measured

  .......

long estimatedTime = System.currentTimeMillis() - startTime;

System.nanoTime(): Returns the current value of the most precise available system timer, in nanoseconds, in long. Introduced in JDK 1.5. nanoTime() is meant for measuring relative time interval instead of providing absolute timing.

// Measuring elapsed time

long startTime = System.nanoTime();

// The code being measured

  .......

long estimatedTime = System.nanoTime() - startTime;

Take note that milli is 10^-3=0.001, nano is 10^-9=0.000000001. There is no micro (10^-6=0.000001) timer.

REFERENCES & RESOURCES

Java Tutorial, Internationalization ⇒ Formatting ⇒ Dates and Times at http://java.sun.com/docs/books/tutorial/i18n/format/dateintro.html.
JDK source codes: java.util.Date, java.util.Calendar, java.util.GregorianCalendar.

Latest version tested: JDK 1.6
Last modified: September, 2010

Feedback, comments, corrections, and errata can be sent to Chua Hock-Chuan (ehchua@ntu.edu.sg) | HOME