Subtract 2 Dates

[mike rich]

I need to calculate the number of days between 2 dates. I think I have confused myself beyond repair and I am seeking some advice. Below is a snip of my code. In a nutshell I am reading 2 string values, converting them to date and then I want to calculate the difference. any help would be appreciated.....
String ds1 = a[x].getinvoice().substring(date_start, last_paren);
String ds2 = a[x].getdate().trim();

DateFormat dt_parse1 = DateFormat.getDateInstance((DateFormat.SHORT));
DateFormat dt_parse2 = DateFormat.getDateInstance((DateFormat.SHORT));

try
{
Date d1 = dt_parse1.parse(ds1);
Date d2 = dt_parse1.parse(ds2);

SimpleDateFormat local_format1 = new SimpleDateFormat("MM/dd/yy");
SimpleDateFormat local_format2 = new SimpleDateFormat("MM/dd/yy");

System.out.println("New Date1: " + local_format1.format(d1));
System.out.println("New Date2: " + local_format1.format(d2));

/*Gregorian*/Calendar cal1 = new GregorianCalendar();
cal1.setTime(d1);

/*Gregorian*/Calendar cal2 = new GregorianCalendar();
cal1.setTime(d2);

System.out.println("New Date1 as calendar object: " + local_format1.format(cal1.getTime()));
System.out.println("New Date2 as calendar object: " + local_format1.format(cal2.getTime()));
System.out.println("********** " + (local_format1.format(cal1.getTime()) - local_format1.format(cal2.getTime())));


}
catch (ParseException p)
{
p.printStackTrace();
}

[Dave Vick]

mike r
Welcome to the Java Ranch, we hope you�ll enjoy visiting as a regular however,
your name is not in keeping with our naming policy here at the ranch. Please change your display name to an appropriate name as shown in the policy.
Thanks again and we hope to see you around the ranch!!

[Marilyn de Queiroz]

I would use some of the methods in the Date class.

[Frank Carver]

It's really important to step back and think about the data types in this problem. It's a surprisingly common one.
What is the conceptual data type of the difference between two dates? If you were making a class to hold the difference between two dates, what would you call it?
Hint: it's not a Date.

[Snigdha Solanki]

Check out the following code:

 
package Dates;
import java.io.Serializable;
/**
 * This class represents a specific day in time.
 *
 * @author  Premanand C.
 * @version 1.0, 03/22/2002
 */
public class Date extends Object implements Comparable, Serializable {
    
    /**
     * A constant holding the minimum year value.
     */
    public static final int MIN_YEAR = 1901;
    
    /**
     * A constant holding the maximum year value.
     */
    public static final int MAX_YEAR = 2099;
    
    /* Array to hold the days in a month */
    private static final int monthDays[] = {0, 31, 28, 31, 30, 31, 30,
    31, 31, 30, 31, 30, 31};
    /* Array to hold the names of the months */
    private static final String []monthName = {null,
          "January", "February", "March", "April", "May", "June", "July",
          "August", "September", "October", "November", "December"};
    private int month; // Month component of the current date.
    private int day;   // Day component of the current date.
    private int year;  // Year component of the current.
    
    /**
     * Denotes the first date of the system.
     */
    protected static Date EPOCH = new Date(1, 1, MIN_YEAR);
    
    /**
     * Returns true if the input is a leap year.
     *
     * @return  a string representation of this date. 
     */
    public static boolean isLeapYear(int year) {
        if ((year % 4 != 0) || (((year % 100) == 0) && (year % 400 != 0)))
            return false;
        return true;
    }
    
    /**
     * Constructs a date object initialized with the given month, day and
     * year.
     *
     * @param month the month component of the date.
     * @param day the day component of the date.
     * @param day the year component of the date.
     *
     * @exception IllegalArgumentException. For invalid parameters.
     */
    public Date(int month, int day, int year)
    throws IllegalArgumentException {
        
        /* Validate the inputs */
        if ((year < MIN_YEAR) || (year > MAX_YEAR))
            throw new IllegalArgumentException("Illegal year " + year + ".");
        
        if ((month <= 0) || (month > 12))
            throw new IllegalArgumentException("Illegal month " + month + ".");
        
        if ((day <= 0) || (isLeapYear(year) && (month == 2) && (day > 29))
        || (day > monthDays[month]))
            throw new IllegalArgumentException("Illegal day " + day + ".");
        
        this.day   = day;
        this.month = month;
        this.year  = year;
        
    }
    
    /**
     * Returns the maximum mumber of days in the current month.
     *
     * @return  <code>true</code> if the value represented by this object is
     *          positive infinity or negative infinity;
     *          <code>false</code> otherwise.
     */
    public int getMaxDaysInMonth() {
        if (isLeapYear(year) && (month == 2))
            return monthDays[month] + 1;
        return monthDays[month];
    }
    
    /**
     * Compares two Dates for ordering.
     *
     * @param   anotherDate   the <code>Date</code> to be compared.
     * @return  the value <code>0</code> if the argument Date is equal to
     *          this Date; a value less than <code>0</code> if this Date
     *          is before the Date argument; and a value greater than
     *      <code>0</code> if this Date is after the Date argument.
     */
    public int compareTo(Date anotherDate) {
        
        if (this.year != anotherDate.year)
            return ((this.year - anotherDate.year > 0) ? 1 : -1);
            
        if (this.month != anotherDate.month)
            return ((this.month - anotherDate.month > 0) ? 1 : -1);
                
        if (this.day != anotherDate.day)
            return ((this.day - anotherDate.day > 0) ? 1 : -1);
        return 0;
    }
    
    /**
     * Compares this Date to another Object.  If the Object is a Date,
     * this function behaves like <code>compareTo(Date)</code>.  Otherwise,
     * it throws a <code>ClassCastException</code> (as Dates are comparable
     * only to other Dates).
     *
     * @param   o the <code>Object</code> to be compared.
     * @return  the value <code>0</code> if the argument is a Date
     *      equal to this Date; <code>-1</code> if the
     *      argument is a Date after this Date; and <code>1</code>
     *      if the argument is a Date before this Date.
     * @exception ClassCastException if the argument is not a
     *        <code>Date</code>. 
     * @see     java.lang.Comparable
     */
    public int compareTo(Object anotherDate) {
        return compareTo((Date)anotherDate);
    }
    
    /**
     * Compares two dates for equality.
     * The result is <code>true</code> if and only if the argument is 
     * not <code>null</code> and is a <code>Date</code> object that 
     * represents the same date as this object.
     * <p>
     *
     * @param   anotherDate   the object to compare with.
     * @return  <code>true</code> if the dates are the same;
     *          <code>false</code> otherwise.
     */
    public boolean equals(Object anotherDate) {
        
        if (!(anotherDate instanceof Date) || anotherDate == null)
            return false;
        
        Date date = (Date)anotherDate;
        
        if ((this.year != date.year) || (this.month != date.month) || (this.day !=
        date.day))
            return false;
        return true;
    }
    
    /**
     * Returns the number of days elapsed since the Epoch, 1st January 1901.
     *
     * @return  the number of days since the Epoch.
     */
    public int getDaysFromEpoch() {
        int i;
        int days = 0;
        for(i = EPOCH.year; i < this.year; i++)
            days += isLeapYear(i) ? 366 : 365;
        for(i = EPOCH.month; i < this.month; i++)
            days += monthDays[i];
            
        days += this.day - EPOCH.day;
        return days;
    }
    
    /**
     * Converts this <code>Date</code> object to a <code>String</code> 
     *
     * @return  a string representation of this date. 
     */
    public String toString() {
        return (this.day + " " + monthName[this.month] + " " + this.year);
    }
}
 

 
package Dates;
/**
 * This class computes the number of days between two dates.
 * Only the days in between are counted, not the end dates themselves.
 * @author Premanand C.
 * @version 1.0 03/22/2002
 */
public class DateDifference {
    
    private Date date1, date2;
    
    
  /**
   * Constructor that takes two <code> Date </code> arguments. The order in
   * which the dates are entered is not important.
   *
   * @param   date1   First date
   * @param   date2   Second date
   * @exception IllegalArgumentException. For invalid parameters.
   *
   */    public DateDifference(Date date1, Date date2) {
        if ((date1 == null) || (date2 == null))
            throw new NullPointerException("Received null inputs!!");
        this.date1 = date1;
        this.date2 = date2;
    }
    
  /**
   * DateDifference.
   * Constructor that takes six arguments. The first three arguments represent
   * the first date and the last three arguments represent the second date.
   * The order in which the dates are specified is not important.
   *
   * @param   month1  First month of first date
   * @param   day1    First day of first date
   * @param   year1   First year of first date
   * @param   month2  Second month of second date
   * @param   day2    Second day of second date
   * @param   year2   Second year of second date
   * @exception IllegalArgumentException. For invalid parameters.
   */
    public DateDifference(int month1, int day1, int year1,
                               int month2, int day2, int year2) {
        date1 = new Date(month1, day1, year1);
        date2 = new Date(month2, day2, year2);
    }
    
  /**
   * getDaysBetweenDates.
   * Determines the difference in the number of days between the two dates.
   * The begin and end dates themselves are not included in the calculation.
   * For example, 01,01, 2002 and 01,03, 2002 only returns 1 day in between.
   *
   * @return  the number of days between the two dates. The value
   * returned is never negative.
   */
    public int getDaysBetweenDates() {
        int diff = Math.abs(date1.getDaysFromEpoch()
        - date2.getDaysFromEpoch()) - 1;
        return (diff < 0) ? 0 : diff;
    }
}
 

 
package Dates;
import java.util.StringTokenizer;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.io.IOException;
/**
 * This class is a Java application that allows the user to enter two dates and
 * find out the number of days in between them, not including the low and high
 * dates themselves.
 *
 * @author Premanand C.
 * @version 1.0 03/22/2002
 */
public class DateTester extends java.lang.Object {
    /** Creates new DateTester */
    private DateTester() {
    }
    /**
     * The entry point into the class.
     * 
     * @param args the string arguments to the class. Any parameters are
     * not used.
     */
    public static void main (String args[]) {
        
        String inputString      = "";
        DateDifference dateDiff = null;
        StringTokenizer token   = null;
        int m1, y1, d1;
        int m2, y2, d2;
        
        /* Set up a reader to read line-wise from the standard input */
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        char answer = 'Y';
        while (answer == 'Y' || answer == 'y') {
            
            inputString = "";
            dateDiff    = null;
            token       = null;
            m1 = y1 = d1 = 0;
            m2 = y2 = d2 = 0;
            System.out.print("Enter 2 dates in the format mm, dd, yyyy," +
                             " mm, dd, yyyy: ");
            try {
                inputString = in.readLine();
            }
            catch(IOException e) {
                System.err.println("I/O error occurred while getting " +
                                   "input string");
            }
            
            if (inputString != null) {
                token = new StringTokenizer(inputString, ",");
                if (token.countTokens() != 6) {
                    System.err.println("Invalid number of data fields entered."
                                    +  " Please re-enter.");
                }
                else {
                    try {
                        m1 = Integer.parseInt(token.nextToken().trim());
                        d1 = Integer.parseInt(token.nextToken().trim());
                        y1 = Integer.parseInt(token.nextToken().trim());
                        m2 = Integer.parseInt(token.nextToken().trim());
                        d2 = Integer.parseInt(token.nextToken().trim());
                        y2 = Integer.parseInt(token.nextToken().trim());
                        try {
                          dateDiff = new DateDifference(m1, d1, y1, m2, d2, y2);
                          System.out.println("Difference in days: "
                                          + dateDiff.getDaysBetweenDates());
                        } catch (IllegalArgumentException e) {
                            System.out.println(String.valueOf(e.getMessage()));
                        }
                    }
                    catch(NumberFormatException e) {
                        System.err.println("Invalid number format. Please re-enter.");
                    }
                }
            }
            System.out.print("\nDo you want to input once more(Y/N)? ");
            try {
                answer = in.readLine().charAt(0);
            }
            catch(IOException ioe) {
                System.err.println("IO exception when reading answer.");
            }
            catch(NullPointerException npe) {
            }
        }
    }
}
 

where the input dates have format month, day,
year eg ( 6 , 2, 1983 means June 2, 1983)