由于这里所有的答案都是正确的，但使用传统java或第三方库，如joda或类似的，我将放弃使用新java的另一种方式。Java 8及以后版本中的时间类。参见Oracle教程。

使用LocalDate和ChronoUnit:

LocalDate d1 = LocalDate.of(2017, 5, 1);
LocalDate d2 = LocalDate.of(2017, 5, 18);

long days = ChronoUnit.DAYS.between(d1, d2);
System.out.println( days );

如果你想修复跨越夏时制边界的日期范围的问题(例如，一个日期在夏季，另一个日期在冬季)，你可以使用这个来获得天数的差异:

public static long calculateDifferenceInDays(Date start, Date end, Locale locale) {
    Calendar cal = Calendar.getInstance(locale);

    cal.setTime(start);
    cal.set(Calendar.HOUR_OF_DAY, 0);
    cal.set(Calendar.MINUTE, 0);
    cal.set(Calendar.SECOND, 0);
    cal.set(Calendar.MILLISECOND, 0);
    long startTime = cal.getTimeInMillis();

    cal.setTime(end);
    cal.set(Calendar.HOUR_OF_DAY, 0);
    cal.set(Calendar.MINUTE, 0);
    cal.set(Calendar.SECOND, 0);
    cal.set(Calendar.MILLISECOND, 0);
    long endTime = cal.getTimeInMillis();

    // calculate the offset if one of the dates is in summer time and the other one in winter time
    TimeZone timezone = cal.getTimeZone();
    int offsetStart = timezone.getOffset(startTime);
    int offsetEnd = timezone.getOffset(endTime);
    int offset = offsetEnd - offsetStart;

    return TimeUnit.MILLISECONDS.toDays(endTime - startTime + offset);
}

不使用标准API，不行。你可以这样做:

class Duration {
    private final TimeUnit unit;
    private final long length;
    // ...
}

或者你可以使用Joda:

DateTime a = ..., b = ...;
Duration d = new Duration(a, b);

使用GMT时区获取一个Calendar实例，使用Calendar类的set方法设置时间。GMT时区偏移量为0(并不重要)，夏令时标志设置为false。

    final Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));

    cal.set(Calendar.YEAR, 2011);
    cal.set(Calendar.MONTH, 9);
    cal.set(Calendar.DAY_OF_MONTH, 29);
    cal.set(Calendar.HOUR, 0);
    cal.set(Calendar.MINUTE, 0);
    cal.set(Calendar.SECOND, 0);
    final Date startDate = cal.getTime();

    cal.set(Calendar.YEAR, 2011);
    cal.set(Calendar.MONTH, 12);
    cal.set(Calendar.DAY_OF_MONTH, 21);
    cal.set(Calendar.HOUR, 0);
    cal.set(Calendar.MINUTE, 0);
    cal.set(Calendar.SECOND, 0);
    final Date endDate = cal.getTime();

    System.out.println((endDate.getTime() - startDate.getTime()) % (1000l * 60l * 60l * 24l));

public static String getDifferenceBtwTime(Date dateTime) {

    long timeDifferenceMilliseconds = new Date().getTime() - dateTime.getTime();
    long diffSeconds = timeDifferenceMilliseconds / 1000;
    long diffMinutes = timeDifferenceMilliseconds / (60 * 1000);
    long diffHours = timeDifferenceMilliseconds / (60 * 60 * 1000);
    long diffDays = timeDifferenceMilliseconds / (60 * 60 * 1000 * 24);
    long diffWeeks = timeDifferenceMilliseconds / (60 * 60 * 1000 * 24 * 7);
    long diffMonths = (long) (timeDifferenceMilliseconds / (60 * 60 * 1000 * 24 * 30.41666666));
    long diffYears = (long)(timeDifferenceMilliseconds / (1000 * 60 * 60 * 24 * 365));

    if (diffSeconds < 1) {
        return "one sec ago";
    } else if (diffMinutes < 1) {
        return diffSeconds + " seconds ago";
    } else if (diffHours < 1) {
        return diffMinutes + " minutes ago";
    } else if (diffDays < 1) {
        return diffHours + " hours ago";
    } else if (diffWeeks < 1) {
        return diffDays + " days ago";
    } else if (diffMonths < 1) {
        return diffWeeks + " weeks ago";
    } else if (diffYears < 12) {
        return diffMonths + " months ago";
    } else {
        return diffYears + " years ago";
    }
}   

我喜欢基于timeunit的方法，直到我发现它只覆盖了一个时间单元在下一个更高单位中有多少个单位是固定的这种微不足道的情况。当你想知道间隔了多少个月、多少年等时，这个问题就不成立了。

这里有一种计数方法，不像其他方法那么有效，但它似乎对我有用，而且还考虑到了夏令时。

public static String getOffsetAsString( Calendar cNow, Calendar cThen) {
    Calendar cBefore;
    Calendar cAfter;
    if ( cNow.getTimeInMillis() < cThen.getTimeInMillis()) {
        cBefore = ( Calendar) cNow.clone();
        cAfter = cThen;
    } else {
        cBefore = ( Calendar) cThen.clone();
        cAfter = cNow;
    }
    // compute diff
    Map<Integer, Long> diffMap = new HashMap<Integer, Long>();
    int[] calFields = { Calendar.YEAR, Calendar.MONTH, Calendar.DAY_OF_MONTH, Calendar.HOUR_OF_DAY, Calendar.MINUTE, Calendar.SECOND, Calendar.MILLISECOND};
    for ( int i = 0; i < calFields.length; i++) {
        int field = calFields[ i];
        long    d = computeDist( cAfter, cBefore, field);
        diffMap.put( field, d);
    }
    final String result = String.format( "%dY %02dM %dT %02d:%02d:%02d.%03d",
            diffMap.get( Calendar.YEAR), diffMap.get( Calendar.MONTH), diffMap.get( Calendar.DAY_OF_MONTH), diffMap.get( Calendar.HOUR_OF_DAY), diffMap.get( Calendar.MINUTE), diffMap.get( Calendar.SECOND), diffMap.get( Calendar.MILLISECOND));
    return result;
}

private static int computeDist( Calendar cAfter, Calendar cBefore, int field) {
    cBefore.setLenient( true);
    System.out.print( "D " + new Date( cBefore.getTimeInMillis()) + " --- " + new Date( cAfter.getTimeInMillis()) + ": ");
    int count = 0;
    if ( cAfter.getTimeInMillis() > cBefore.getTimeInMillis()) {
        int fVal = cBefore.get( field);
        while ( cAfter.getTimeInMillis() >= cBefore.getTimeInMillis()) {
            count++;
            fVal = cBefore.get( field);
            cBefore.set( field, fVal + 1);
            System.out.print( count + "/"  + ( fVal + 1) + ": " + new Date( cBefore.getTimeInMillis()) + " ] ");
        }
        int result = count - 1;
        cBefore.set( field, fVal);
        System.out.println( "" + result + " at: " + field + " cb = " + new Date( cBefore.getTimeInMillis()));
        return result;
    }
    return 0;
}