Java Date, Calendar and Time API

 

Java Date and Time API

This article explains the API for using Calendar, Date and Time in Java and how to format the output of a date.

 

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Table of Contents

1. Overview

2. Format date

3. Working with Dates and Calendars

3.1. Calendar

3.2. Date and Date Conversion

4. About this website

4.1.

5. Links and Literature

1. Overview

The Java language provides direct support for time-based objects. This article gives a few examples how this API can be used.

The java.util.Date and the java.util.Calendar classes provide access to storing and manipulating dates.

It is recommended to use Calendar if possible. Existing API may required that you convert from Date to Calendar and vice versa.

 

2. Format date

To format a date, you can use the SimpleDateFormat class. The following snippet gives several example for its usage.

// use dd/MM/yy as format

DateFormat df1 = new SimpleDateFormat("dd/MM/yy");
String formattedDate1 = df1.format(new Date());

// or use yyyy/MM/dd as format

DateFormat df2 = new SimpleDateFormat("yyyy/MM/dd");
String formattedDate2 = df2.format(theDate); 
 

　　

3. Working with Dates and Calendars

3.1. Calendar

The java.util.Calendar class is an abstract encapsulation of the Date object.

Calendar provides getter and setter for the date fields.

public final int get(int field)
public final void set(int field, int value) 
 

　　

Table 1. Calendar field access

Field	Explanation
Calendar.YEAR	Identifies the year
Calendar.MONTH	Identifies the month
Calendar.DAY_OF_MONTH	Identifies the day
Calendar.HOUR	Identifies the hour
Calendar.MINUTE	Identifies the minute
Calendar.SECOND	Identifies the second
 

　　

Tip

 The Calendar.MONTH starts with 0. So December is 11.

Create a new Java project called JavaIntroCalendar. Create the following class for testing.

 

package test;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.GregorianCalendar;

public class CalendarTest {
  public static void main(String[] args) {
    // constructor allows to set year, month and date
    Calendar cal1 = new GregorianCalendar(2008, 01, 01);
    // constructor could also be empty
    // calendar cal2 = new GregorianCalendar();
    // change the month
    cal1.set(Calendar.MONTH, Calendar.MAY);

    System.out.println("Year: " + cal1.get(Calendar.YEAR));
    System.out.println("Month: " + (cal1.get(Calendar.MONTH) + 1));
    System.out.println("Days: " + cal1.get(Calendar.DAY_OF_MONTH));

    // format the output with leading zeros for days and month
    SimpleDateFormat date_format = new SimpleDateFormat("yyyyMMdd");
    System.out.println(date_format.format(cal1.getTime()));

  }
} 
 

　　

3.2. Date and Date Conversion

Use the following commands to convert to a Date from various formats.

package conversion;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

public class ConversionExamplesDate {

  // convert from String to date
  private void stringToDate() {
    
    try {
      Date date1;
      date1 = new SimpleDateFormat("MM/dd/yy").parse("05/18/05");
      System.out.println(date1);
      Date date2 = new SimpleDateFormat("MM/dd/yyyy").parse("05/18/2007");
      System.out.println(date2);
    } catch (ParseException e) {
      e.printStackTrace();
    }
  }

  // convert from millisecs to a String with a defined format
  private void calcDate(long millisecs) {
    SimpleDateFormat date_format = new SimpleDateFormat("MMM dd,yyyy HH:mm");
    Date resultdate = new Date(millisecs);
    System.out.println(date_format.format(resultdate));
  }
  
  private void writeActualDate(){
    Calendar cal = new GregorianCalendar();
    Date creationDate = cal.getTime();
    SimpleDateFormat date_format = new SimpleDateFormat("MMM dd,yyyy HH:mm");
    System.out.println(date_format.format(creationDate));
  }
  

  public static void main(String[] args) {
    ConversionExamplesDate convert = new ConversionExamplesDate();
    convert.stringToDate();
    convert.calcDate(System.currentTimeMillis());
    convert.writeActualDate();
  }
}