不幸的是，JDK Date API被严重破坏了。我推荐使用Joda Time库。

Joda Time有一个时间间隔的概念:

Interval interval = new Interval(oldTime, new Instant());

编辑:顺便说一下，Joda有两个概念:Interval表示两个时间瞬间之间的时间间隔(表示上午8点到上午10点之间的时间)，Duration表示没有实际时间边界的时间长度(例如表示两个小时!)

如果你只关心时间比较，大多数Date实现(包括JDK)实现了Comparable接口，允许你使用Comparable. compareto ()

下面的代码可以给你想要的输出:

String startDate = "Jan 01 2015";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MMM dd yyyy");
LocalDate date = LocalDate.parse(startDate, formatter);

String currentDate = "Feb 11 2015";
LocalDate date1 = LocalDate.parse(currentDate, formatter);

System.out.println(date1.toEpochDay() - date.toEpochDay());

先回答最初的问题:

将以下代码放入Long getAge(){}这样的函数中

Date dahora = new Date();
long MillisToYearsByDiv = 1000l *60l * 60l * 24l * 365l;
long javaOffsetInMillis = 1990l * MillisToYearsByDiv;
long realNowInMillis = dahora.getTime() + javaOffsetInMillis;
long realBirthDayInMillis = this.getFechaNac().getTime() + javaOffsetInMillis;
long ageInMillis = realNowInMillis - realBirthDayInMillis;

return ageInMillis / MillisToYearsByDiv;

这里最重要的是在乘法和除法时处理长数字。当然，还有Java在日期演算中应用的偏移量。

:)

因为这个问题用Scala做了标记，

import scala.concurrent.duration._
val diff = (System.currentTimeMillis() - oldDate.getTime).milliseconds
val diffSeconds = diff.toSeconds
val diffMinutes = diff.toMinutes
val diffHours = diff.toHours
val diffDays = diff.toDays

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 );

另一个纯Java变体:

public boolean isWithin30Days(Calendar queryCalendar) {

    // 1. Take the date you are checking, and roll it back N days
    Calendar queryCalMinus30Days = Calendar.getInstance();
    queryCalMinus30Days.setTime(queryCalendar.getTime());
    queryCalMinus30Days.add(Calendar.DATE, -30); // subtract 30 days from the calendar

    // 2. Get respective milliseconds for the two Calendars: now & queryCal minus N days 
    long nowL = Calendar.getInstance().getTimeInMillis();
    long queryCalMinus30DaysL = queryCalMinus30Days.getTimeInMillis();

    // 3. if nowL is still less than the queryCalMinus30DaysL, it means queryCalendar is more than 30 days into future
    boolean isWithin30Days = nowL >= queryCalMinus30DaysL;

    return isWithin30Days;
}

感谢这里的入门代码:https://stackoverflow.com/a/30207726/2162226