How to get past date ?

[lily ling]

Hi, there
I want to get the date which is 80 days before certain date ( let's say today ), may I ask how to compute that date ?
Thanks in advance,
Lily

[john guthrie]

I'd use simple math:
long now = System.currentTimeMillis();
long eightyDays = (long)80 * 24 * 60 * 60 * 1000;
Date then = new Date(now-eightyDays);

[Neil Laurance]

Or you can use the Calendar class:

// Ago.java
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.GregorianCalendar;
public class Ago {
  public static void main(String[] args) {
    DateFormat format = new SimpleDateFormat("dd-MM-yyyy");
    Calendar date = new GregorianCalendar();
    System.out.println( format.format(date.getTime()) );
    date.add(Calendar.DATE, -80);
    System.out.println( format.format(date.getTime()) );
    date.add(Calendar.DATE, -200);
    System.out.println( format.format(date.getTime()) );
  }
}

Cheers, Neil

[lily ling]

Thanks a lot.
Yes, by using Calendar ( the second way ), I could print out the time I want ( 80 days ago, let's say ). If I want to know if a Date obj passed in is before this Calendar date, how to convert that Date obj to a Calendar obj so that I could use CalendarObj.before( passed_in_Date_to_CalendarObj ) to do the comparison ?
Tons of thanks,
Lily

[Neil Laurance]

The Calendar instance method getTime() returns a java.util.Date so you readily compare the 2, for example:

// Ago.java
import java.util.Date;
import java.util.Calendar;
import java.util.GregorianCalendar;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
public class Ago {
  public static void main(String[] args) {
    DateFormat format = new SimpleDateFormat("dd-MM-yyyy");
    Calendar date1 = new GregorianCalendar();
    Calendar date2 = (Calendar)date1.clone();
    date2.add(Calendar.DATE, -80);
    // comparing Calendar objects
    System.out.println(
        format.format(date2.getTime()) + " before " +
        format.format(date1.getTime()) + " = " +
        date2.before(date1));
    // comparing Calendar and Date
    Date today = new Date();
    System.out.println(
        format.format(date2.getTime()) + " before " +
        format.format(today) + " = " +
        date2.getTime().before(today));
  }
}

Returns:

21-05-2002 before 09-08-2002 = true
21-05-2002 before 09-08-2002 = true

Cheers Neil

[lily ling]

Thanks a lot, Neil and John. I got all I wanted.
Lily