add() and roll() are used to add or substract values to a Calendar object.

You specify which Calendar field is to be affected by the operation (Calendar.YEAR, Calendar.MONTH, Calendar.DATE).

add() adds or substracts values to the specified Calendar field, the next larger field is modified when the result makes the Calendar "rolls over".

 String DATE_FORMAT = "yyyy-MM-dd";
java.text.SimpleDateFormat sdf = 
      new java.text.SimpleDateFormat(DATE_FORMAT);
Calendar c1 = Calendar.getInstance(); 
c1.set(1999, 0 , 20); // 1999 jan 20
System.out.println("Date is : " + sdf.format(c1.getTime()));
c1.add(Calendar.DATE,20);
System.out.println("Date + 20 days is : " + sdf.format(c1.getTime()));

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.

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

c1.set(1999, 0 , 20); // 1999 jan 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())); // 1998 dec 20