Note: this answer was written in 2011. I would recommend using java.time now instead of Joda Time.

Well to start with, you should only deal with them as strings when you have to. Most of the time you should work with them in a data type which actually describes the data you're working with.

I would recommend that you use Joda Time, which is a much better API than Date/Calendar. It sounds like you should use the LocalDate type in this case. You can then use:

int days = Days.daysBetween(date1, date2).getDays();

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The original answer from 2013 is now outdated because some of the classes have been replaced.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Original answer (outdated as of Java 8)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.

If you want logical calendar days, use DAYS.between() method from java.time.temporal.ChronoUnit:

LocalDate dateBefore;
LocalDate dateAfter;
long daysBetween = ChronoUnit.DAYS.between(dateBefore, dateAfter);

If you want literal 24 hour days, (a duration), you can use the Duration class instead:

LocalDate today = LocalDate.now()
LocalDate yesterday = today.minusDays(1);
// Duration oneDay = Duration.between(today, yesterday); // throws an exception
Duration.between(today.atStartOfDay(), yesterday.atStartOfDay()).toDays() // another option

For more information, refer to this document: Java SE 8 Date and Time.

try this:

int epoch = (int) (new java.text.SimpleDateFormat("MM/dd/yyyy HH:mm:ss").parse("01/01/1970  00:00:00").getTime() / 1000);

you can edit the string in the parse() methods param.