UPDATE

The original answer from 2013 is now outdated because some of the classes have been replaced. The new way of doing this is using the new java.time classes.

24-hour days

DateTimeFormatter dtf = DateTimeFormatter.ofPattern("dd MM yyyy");
String inputString1 = "23 01 1997";
String inputString2 = "27 04 1997";

try {
    LocalDateTime date1 = LocalDate.parse(inputString1, dtf).atStartOfDay();
    LocalDateTime date2 = LocalDate.parse(inputString2, dtf).atStartOfDay();
    long daysBetween = Duration.between(date1, date2).toDays();
    System.out.println ("Days: " + daysBetween);
} catch (DateTimeParseException e) {
    e.printStackTrace();
}

Calendar days

Note that the solution above counts days as generic chunks of 24 hours, not calendar days.

For calendar days, use java.time.temporal.ChronoUnit.DAYS and its between method.

long daysBetween = ChronoUnit.DAYS.between(date1, date2) ;

Original answer (outdated as of Java 8)

You are making some conversions with your Strings that are not necessary. There is a SimpleDateFormat class for it - try this:

SimpleDateFormat myFormat = new SimpleDateFormat("dd MM yyyy");
String inputString1 = "23 01 1997";
String inputString2 = "27 04 1997";

try {
    Date date1 = myFormat.parse(inputString1);
    Date date2 = myFormat.parse(inputString2);
    long diff = date2.getTime() - date1.getTime();
    System.out.println ("Days: " + TimeUnit.DAYS.convert(diff, TimeUnit.MILLISECONDS));
} catch (ParseException e) {
    e.printStackTrace();
}

There have been some discussions regarding the correctness of this code. It does indeed take care of leap years. However, the TimeUnit.DAYS.convert function loses precision since milliseconds are converted to days (see the linked doc for more info). If this is a problem, diff can also be converted by hand:

float days = (diff / (1000*60*60*24));

Note that this is a float value, not necessarily an int.

Use something like:

Date date; // your date
// Choose time zone in which you want to interpret your Date
Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("Europe/Paris"));
cal.setTime(date);
int year = cal.get(Calendar.YEAR);
int month = cal.get(Calendar.MONTH);
int day = cal.get(Calendar.DAY_OF_MONTH);
// etc.

Beware, months start at 0, not 1.

Edit: Since Java 8 it's better to use java.time.LocalDate rather than java.util.Calendar. See this answer for how to do it.

Something like this should do the trick:

String dt = "2008-01-01";  // Start date
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Calendar c = Calendar.getInstance();
c.setTime(sdf.parse(dt));
c.add(Calendar.DATE, 1);  // number of days to add
dt = sdf.format(c.getTime());  // dt is now the new date

Yes. Depending on your exact case:

• You can use java.util.Calendar:

  Calendar c = Calendar.getInstance();
  c.setTime(yourDate);
  int dayOfWeek = c.get(Calendar.DAY_OF_WEEK);
  

• if you need the output to be Tue rather than 3 (Days of week are indexed starting at 1 for Sunday, see Calendar.SUNDAY), instead of going through a calendar, just reformat the string: new SimpleDateFormat("EE").format(date) (EE meaning "day of week, short version")

• if you have your input as string, rather than Date, you should use SimpleDateFormat to parse it: new SimpleDateFormat("dd/M/yyyy").parse(dateString)

• you can use joda-time's DateTime and call dateTime.dayOfWeek() and/or DateTimeFormat.

• edit: since Java 8 you can now use java.time package instead of joda-time

SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
Calendar c = Calendar.getInstance();
c.setTime(new Date()); // Using today's date
c.add(Calendar.DATE, 5); // Adding 5 days
String output = sdf.format(c.getTime());
System.out.println(output);

You don't have to use Calendar. You can just play with timestamps :

Date d = initDate();//intialize your date to any date 
Date dateBefore = new Date(d.getTime() - n * 24 * 3600 * 1000 l ); //Subtract n days   

UPDATE DO NOT FORGET TO ADD "l" for long by the end of 1000.

Please consider the below WARNING:

Adding 1000*60*60*24 milliseconds to a java date will once in a great while add zero days or two days to the original date in the circumstances of leap seconds, daylight savings time and the like. If you need to be 100% certain only one day is added, this solution is not the one to use.

Given a Date dt you have several possibilities:

Solution 1: You can use the Calendar class for that:

Date dt = new Date();
Calendar c = Calendar.getInstance(); 
c.setTime(dt); 
c.add(Calendar.DATE, 1);
dt = c.getTime();

Solution 2: You should seriously consider using the Joda-Time library, because of the various shortcomings of the Date class. With Joda-Time you can do the following:

Date dt = new Date();
DateTime dtOrg = new DateTime(dt);
DateTime dtPlusOne = dtOrg.plusDays(1);

Solution 3: With Java 8 you can also use the new JSR 310 API (which is inspired by Joda-Time):

Date dt = new Date();
LocalDateTime.from(dt.toInstant()).plusDays(1);

Solution 4: With org.apache.commons.lang3.time.DateUtils you can do:

Date dt = new Date();
dt = DateUtils.addDays(dt, 1)