Calendar and Date

Formatting dates with the DateFormat class is a two-step process. First, you create a formatter with the getDateInstance method. Second, you invoke the format method, which returns a String containing the formatted date.

The following example formats today's date by calling these two methods:

Date today;
String dateOut;
DateFormat dateFormatter;

dateFormatter = DateFormat.getDateInstance(DateFormat.DEFAULT, currentLocale);
today = new Date();
dateOut = dateFormatter.format(today);
System.out.println(dateOut + " " + currentLocale.toString());

Prior to JDK 1.1, the class Date had two additional functions. It allowed the interpretation of dates as year, month, day, hour, minute, and second values. It also allowed the formatting and parsing of date strings.

Use the Calendar class to convert between dates and time fields. Use the DateFormat class to format and parse date strings. Date is just a wrapper around a long milliseconds since 1970-01-01.

To create a Gregorian calendar with a particular date:
GregorianCalendar gc = new GregorianCalendar(2010,2,15);

• Year must be a four digit integer
• Month is an integer from 0 to 11
• Day must be an integer from 1 to 31

import java.util.*;
public class DateActivity
{
    public static void main(String args[])
    {
        String months[] = 
       {"Jan","Feb","Mar","Apr","May","Jun",
               "Jul","Aug","Sep","Oct","Dec"};

        int year;
        //create a Gregorian calendar with a particular date
        //Year must be a four digit integer
        //Month is an integer from 0 to 11
        //Day must be an integer from 1 to 31
        GregorianCalendar gc = new GregorianCalendar(2010,2,15);
        //Calendar.MONTH is an enumeration that tells gc.get what to return
        System.out.println("Month:" + gc.get(Calendar.MONTH));

        //This will show the month from the array defined above
        System.out.println("Month:" + months[gc.get(Calendar.MONTH)]);
        System.out.println("Day:" + gc.get(Calendar.DATE));
        System.out.println("Year:" + gc.get(Calendar.YEAR));

        //the day of the week is a numeric value from 1 to 7
        System.out.print("Day of Week: ");
        System.out.println(gc.get(Calendar.DAY_OF_WEEK));

        //display the time (you have to set the time above first)
        //GregorianCalendar gc = new GregorianCalendar(2010,2,15,15,30,0);
        System.out.print("Time: ");
        System.out.print(gc.get(Calendar.HOUR)+":");
        System.out.print(gc.get(Calendar.MINUTE) + ":");
        System.out.println(gc.get(Calendar.SECOND));

        //How many days ago was the date?
        //get the milliseconds and convert to the number of days
        long DateInMS = gc.getTimeInMillis();
        //you can also get this from the Date class
        //The Gregorian Calendar .getTime method returns a date object
        //A date object is the number of milliseconds
        //since January 1, 1970
        //The Date class allocates a Date object and initializes it so that it 
        //represents the time at which it was allocated, measured to the nearest millisecond.
        Date myDate = gc.getTime();
        myDate.getTime();

        //once you know the number of milliseconds you can convert them to the number of days
        long millisecondsPerDay = (24*60*60*1000);
        float numberOfDays = DateInMS/millisecondsPerDay;

        System.out.print("Number of Days: ");
        System.out.println(numberOfDays);


    }
}