// date validation using SimpleDateFormat
// it will take a string and make sure it's in the proper
// format as defined by you, and it will also make sure that
// it's a legal date

public boolean isValidDate(String date)
{
   // set date format, this can be changed to whatever format
   // you want, MM-dd-yyyy, MM.dd.yyyy, dd.MM.yyyy etc.
   // you can read more about it here:
   // http://java.sun.com/j2se/1.4.2/docs/api/index.html
   
   SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy");
   
   // declare and initialize testDate variable, this is what will hold
   // our converted string
   
   Date testDate = null;

   // we will now try to parse the string into date form
   try
   {
     testDate = sdf.parse(date);
   }

   // if the format of the string provided doesn't match the format we
   // declared in SimpleDateFormat() we will get an exception

   catch (ParseException e)
   {
     errorMessage = "the date you provided is in an invalid date" +
                             " format.";
     return false;
   }

   // dateformat.parse will accept any date as long as it's in the format
   // you defined, it simply rolls dates over, for example, december 32
   // becomes jan 1 and december 0 becomes november 30
   // This statement will make sure that once the string
   // has been checked for proper formatting that the date is still the
   // date that was entered, if it's not, we assume that the date is invalid

   if (!sdf.format(testDate).equals(date))
   {
     errorMessage = "The date that you provided is invalid.";
     return false;
   }
   
   // if we make it to here without getting an error it is assumed that
   // the date was a valid one and that it's in the proper format

   return true;

} // end isValidDate

<input type="text" value="1900-01-01" name="birthday"  size=20>

if(!isValidDate(birthday))
{
//do something here
}

GregorianCalendar cal = new GregorianCalendar();
cal.setLenient(false);
cal.set(Calendar.YEAR, getBirthYear());
cal.set(Calendar.MONTH, getBirthMonth());
cal.set(Calendar.DAY_OF_MONTH, getBirthDay());

try {
  //This is important!!!! Look at javadoc of calendar
  cal.getTimeInMillis();
} catch (Exception e) {
//will be thrown, if the date is not valid
//handle error
}

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
public class DateUtility {
    
/* Add Day/Month/Year to a Date
add() is used to add  values to a Calendar object.
You specify which Calendar field is to be affected by the operation
(Calendar.YEAR, Calendar.MONTH, Calendar.DATE).
*/
    public static void addToDate(){
        System.out.println("In the ADD Operation");
    //    String DATE_FORMAT = "yyyy-MM-dd";
        String DATE_FORMAT = "dd-MM-yyyy";        //Refer Java DOCS for formats
        java.text.SimpleDateFormat sdf =  new java.text.SimpleDateFormat(DATE_FORMAT);
        Calendar c1 = Calendar.getInstance();
        Date d1 = new Date();
        System.out.println("Todays date in Calendar Format : "+c1);
        System.out.println("c1.getTime() : "+c1.getTime());
        System.out.println("c1.get(Calendar.YEAR): " + c1.get(Calendar.YEAR));
        System.out.println("Todays date in Date Format : "+d1);
        c1.set(1999,0 ,20);         //(year,month,date)
        System.out.println("c1.set(1999,0 ,20) : "+c1.getTime());
        c1.add(Calendar.DATE,40);
        System.out.println("Date + 20 days is : " + sdf.format(c1.getTime()));
        System.out.println();
        System.out.println();
    }
    
    
/*Substract Day/Month/Year to a Date
    roll() is used to substract values to a Calendar object.
    You specify which Calendar field is to be affected by the operation
    (Calendar.YEAR, Calendar.MONTH, Calendar.DATE).
    
    Note: To substract, simply use a negative argument.
    roll() does the same thing except you specify if you want to roll up (add 1)
    or roll down (substract 1) to the specified Calendar field. The operation only
    affects the specified field while add() adjusts other Calendar fields.
    See the following example, roll() makes january rolls to december in the same
    year while add() substract the YEAR field for the correct result

*/
    
    public static void subToDate(){
        System.out.println("In the SUB Operation");
        String DATE_FORMAT = "dd-MM-yyyy";
        java.text.SimpleDateFormat sdf = new java.text.SimpleDateFormat(DATE_FORMAT);
        Calendar c1 = Calendar.getInstance();
        c1.set(1999, 0 , 20);
        System.out.println("Date is : " + sdf.format(c1.getTime()));
        
        // roll down, substract 1 month
        c1.roll(Calendar.MONTH, false);
        System.out.println("Date roll down 1 month : " + sdf.format(c1.getTime()));

        c1.set(1999, 0 , 20);
        System.out.println("Date is : " + sdf.format(c1.getTime()));
        c1.add(Calendar.MONTH, -1);
        // substract 1 month
        System.out.println("Date minus 1 month : " + sdf.format(c1.getTime()));
        System.out.println();
        System.out.println();
    }
    
    public static void daysBetween2Dates(){
        Calendar c1 = Calendar.getInstance();     //new GregorianCalendar();
        Calendar c2 = Calendar.getInstance();     //new GregorianCalendar();
        c1.set(1999, 0 , 20);
        c2.set(1999, 0 , 22);
        System.out.println("Days Between "+c1.getTime()+"\t"+ c2.getTime()+" is");
        System.out.println((c2.getTime().getTime() - c1.getTime().getTime())/(24*3600*1000));
        System.out.println();
        System.out.println();
    }
    
    public static void daysInMonth() {
        Calendar c1 = Calendar.getInstance();     //new GregorianCalendar();
        c1.set(1999, 6 , 20);
        int year = c1.get(Calendar.YEAR);
        int month = c1.get(Calendar.MONTH);
//        int days = c1.get(Calendar.DATE);
        int [] daysInMonths = {31,28,31,30,31,30,31,31,30,31,30,31};
        daysInMonths[1] += DateUtility.isLeapYear(year) ? 1 : 0;
        System.out.println("Days in "+month+"th month for year "+year+" is "+ daysInMonths[c1.get(Calendar.MONTH)]);
        System.out.println();
        System.out.println();
    }
    
    public static void getDayofTheDate() {
        Date d1 = new Date();
        String day = null;
        DateFormat f = new SimpleDateFormat("EEEE");
        try {
         day = f.format(d1);
        }
        catch(Exception e) {
          e.printStackTrace();
        }
        System.out.println("The dat for "+d1+" is "+day);
        System.out.println();
        System.out.println();
    }
    
    public static void validateAGivenDate() {
        String dt = "20011223";  
        String invalidDt = "20031315";
        String dateformat = "yyyyMMdd";  
        Date dt1=null , dt2=null;
        try {    
            SimpleDateFormat sdf = new SimpleDateFormat(dateformat);    
            sdf.setLenient(false);    
            dt1 = sdf.parse(dt);  
            dt2 = sdf.parse(invalidDt);  
            System.out.println("Date is ok = " + dt1 + "(" + dt + ")");    
        }  
        catch (ParseException e) {    
            System.out.println(e.getMessage());
        }  
        catch (IllegalArgumentException e) {    
            System.out.println("Invalid date");    
        }
        System.out.println();
        System.out.println();
    }
    
    public static void compare2Dates(){
        SimpleDateFormat fm = new SimpleDateFormat("dd-MM-yyyy");
        Calendar c1 = Calendar.getInstance();
        Calendar c2 = Calendar.getInstance();
        
        c1.set(2000, 02, 15);
        c2.set(2001, 02, 15);
        
        System.out.print(fm.format(c1.getTime())+" is ");
        if(c1.before(c2)){
            System.out.println("less than "+c2.getTime());
        }else if(c1.after(c2)){
            System.out.println("greater than "+c2.getTime());
        }else if(c1.equals(c2)){
            System.out.println("is equal to "+fm.format(c2.getTime()));
        }
        System.out.println();
        System.out.println();
    }

    public static boolean isLeapYear(int year){
         if((year%100 != 0) || (year%400 == 0)){
             return true;
         }
         return false;
    }

    public static void main(String args[]){
        addToDate();
        subToDate();
        daysBetween2Dates();    //The "right" way would be to compute the julian day number of both dates and then do the substraction.
        daysInMonth();
        validateAGivenDate();
        compare2Dates();
        getDayofTheDate();
    }
    
}