Since dates can contain hours and minutes, final result will be rounded down, which will result in incorrect value. For example, you calculate difference between today at 22:00 p.m and day after tomorrow 00:00 a.m, so the final result will be 1, because in reality it was 1.08 or smth difference, then it gets rounded down when calling TimeUnit.MILLISECONDS.toDays(..). That's why you need to take that in account, so in my solution I subtract the remainder of milliseconds from milliseconds in a day. Additionally, if you want to count the end date, you need to +1 it.

import java.util.Date;
import java.util.concurrent.TimeUnit;

public static long getDaysBetween(Date date1, Date date2, boolean includeEndDate) {
        long millisInDay = 60 * 60 * 24 * 1000;
        long difference = Math.abs(date1.getTime() - date2.getTime());
        long add = millisInDay - (difference % millisInDay);//is used to calculate true number of days, because by default hours, minutes are also counted

        return TimeUnit.MILLISECONDS.toDays(difference + add) + (includeEndDate ? 1 : 0);
    }

测试:

Date date1 = new Date(121, Calendar.NOVEMBER, 27); //2021 Nov 27
Date date2 = new Date(121, Calendar.DECEMBER, 29); //2021 Dec 29
System.out.println( getDaysBetween(date1, date2, false) ); //32 days difference
System.out.println( getDaysBetween(date1, date2, true) ); //33 days difference

以毫秒为单位减去日期是可行的(如另一篇文章所述)，但在清除日期的时间部分时，你必须使用HOUR_OF_DAY而不是HOUR:

public static final long MSPERDAY = 60 * 60 * 24 * 1000;
...
final Calendar dateStartCal = Calendar.getInstance();
dateStartCal.setTime(dateStart);
dateStartCal.set(Calendar.HOUR_OF_DAY, 0); // Crucial.
dateStartCal.set(Calendar.MINUTE, 0);
dateStartCal.set(Calendar.SECOND, 0);
dateStartCal.set(Calendar.MILLISECOND, 0);
final Calendar dateEndCal = Calendar.getInstance();
dateEndCal.setTime(dateEnd);
dateEndCal.set(Calendar.HOUR_OF_DAY, 0); // Crucial.
dateEndCal.set(Calendar.MINUTE, 0);
dateEndCal.set(Calendar.SECOND, 0);
dateEndCal.set(Calendar.MILLISECOND, 0);
final long dateDifferenceInDays = ( dateStartCal.getTimeInMillis()
                                  - dateEndCal.getTimeInMillis()
                                  ) / MSPERDAY;
if (dateDifferenceInDays > 15) {
    // Do something if difference > 15 days
}

这是一个正确的Java 7解决方案，没有任何依赖。

public static int countDaysBetween(Date date1, Date date2) {

    Calendar c1 = removeTime(from(date1));
    Calendar c2 = removeTime(from(date2));

    if (c1.get(YEAR) == c2.get(YEAR)) {

        return Math.abs(c1.get(DAY_OF_YEAR) - c2.get(DAY_OF_YEAR)) + 1;
    }
    // ensure c1 <= c2
    if (c1.get(YEAR) > c2.get(YEAR)) {
        Calendar c = c1;
        c1 = c2;
        c2 = c;
    }
    int y1 = c1.get(YEAR);
    int y2 = c2.get(YEAR);
    int d1 = c1.get(DAY_OF_YEAR);
    int d2 = c2.get(DAY_OF_YEAR);

    return d2 + ((y2 - y1) * 365) - d1 + countLeapYearsBetween(y1, y2) + 1;
}

private static int countLeapYearsBetween(int y1, int y2) {

    if (y1 < 1 || y2 < 1) {
        throw new IllegalArgumentException("Year must be > 0.");
    }
    // ensure y1 <= y2
    if (y1 > y2) {
        int i = y1;
        y1 = y2;
        y2 = i;
    }

    int diff = 0;

    int firstDivisibleBy4 = y1;
    if (firstDivisibleBy4 % 4 != 0) {
        firstDivisibleBy4 += 4 - (y1 % 4);
    }
    diff = y2 - firstDivisibleBy4 - 1;
    int divisibleBy4 = diff < 0 ? 0 : diff / 4 + 1;

    int firstDivisibleBy100 = y1;
    if (firstDivisibleBy100 % 100 != 0) {
        firstDivisibleBy100 += 100 - (firstDivisibleBy100 % 100);
    }
    diff = y2 - firstDivisibleBy100 - 1;
    int divisibleBy100 = diff < 0 ? 0 : diff / 100 + 1;

    int firstDivisibleBy400 = y1;
    if (firstDivisibleBy400 % 400 != 0) {
        firstDivisibleBy400 += 400 - (y1 % 400);
    }
    diff = y2 - firstDivisibleBy400 - 1;
    int divisibleBy400 = diff < 0 ? 0 : diff / 400 + 1;

    return divisibleBy4 - divisibleBy100 + divisibleBy400;
}


public static Calendar from(Date date) {

    Calendar c = Calendar.getInstance();
    c.setTime(date);

    return c;
}


public static Calendar removeTime(Calendar c) {

    c.set(HOUR_OF_DAY, 0);
    c.set(MINUTE, 0);
    c.set(SECOND, 0);
    c.set(MILLISECOND, 0);

    return c;
}

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));

以下是一种解决方案，因为我们有许多方法可以实现这一点:

  import java.util.*; 
   int syear = 2000;
   int eyear = 2000;
   int smonth = 2;//Feb
   int emonth = 3;//Mar
   int sday = 27;
   int eday = 1;
   Date startDate = new Date(syear-1900,smonth-1,sday);
   Date endDate = new Date(eyear-1900,emonth-1,eday);
   int difInDays = (int) ((endDate.getTime() - startDate.getTime())/(1000*60*60*24));

试试这个:

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

你可以在parse()方法参数中编辑字符串。