Java.util.Calendar - 自1970年1月1日以来的毫秒数

Question

Program followed by output. Someone please explain to me why 10,000,000 milliseconds from Jan 1, 1970 is November 31, 1969. Well, someone please explain what's wrong with my assumption that the first test should produce a time 10,000,000 milliseconds from Jan 1, 1970. Numbers smaller than 10,000,000 produce the same result.

public static void main(String[] args) {

    String x = "10000000";
    long l = new Long(x).longValue();
    System.out.println("Long value: " + l);

    Calendar c = new GregorianCalendar();
    c.setTimeInMillis(l);
    System.out.println("Calendar time in Millis: " + c.getTimeInMillis());

    String origDate = c.get(Calendar.YEAR) + "-" + c.get(Calendar.MONTH) + "-" + c.get(Calendar.DAY_OF_MONTH);  
    System.out.println("Date in YYYY-MM-DD format: " + origDate);

    x = "1000000000000";
    l = new Long(x).longValue();
    System.out.println("\nLong value: " + l);

    c.setTimeInMillis(l);
    System.out.println("Calendar time in Millis: " + c.getTimeInMillis());

    origDate = c.get(Calendar.YEAR) + "-" + c.get(Calendar.MONTH) + "-" + c.get(Calendar.DAY_OF_MONTH);  
    System.out.println("Date in YYYY-MM-DD format: " + origDate);
}

Long value: 10000000

Calendar time in Millis: 10000000

Date in YYYY-MM-DD format: 1969-11-31

Long value: 1000000000000

Calendar time in Millis: 1000000000000

Date in YYYY-MM-DD format: 2001-8-8

Answer 1

The dates you print from Calendar are local to your timezone, whereas the epoch is defined to be midnight of 1970-01-01 in UTC. So if you live in a timezone west of UTC, then your date will show up as 1969-12-31, even though (in UTC) it's still 1970-01-01.

Answer 2

First, c.get(Calendar.MONTH) returns 0 for Jan, 1 for Feb, etc.

Second, use DateFormat to output dates.

Third, your problems are a great example of how awkward Java's Date API is. Use Joda Time API if you can. It will make your life somewhat easier.

Here's a better example of your code, which indicates the timezone:

public static void main(String[] args) {

    final DateFormat dateFormat = SimpleDateFormat.getDateTimeInstance(DateFormat.FULL, DateFormat.FULL);

    long l = 10000000L;
    System.out.println("Long value: " + l);
    Calendar c = new GregorianCalendar();
    c.setTimeInMillis(l);
    System.out.println("Date: " + dateFormat.format(c.getTime()));

    l = 1000000000000L;
    System.out.println("\nLong value: " + l);
    c.setTimeInMillis(l);
    System.out.println("Date: " + dateFormat.format(c.getTime()));
}

Answer 3

Calendar#setTimeInMillis() sets the calendar's time to the number of milliseconds after Jan 1, 1970 GMT .

Calendar#get() returns the requested field adjusted for the calendar's timezone which, by default, is your machine's local timezone .

This should work as you expect if you specify "GMT" timezone when you construct the calendar:

Calendar c = new GregorianCalendar(TimeZone.getTimeZone("GMT"));

Answer 4

遗憾的是， java.util.Date和java.util.Calendar的设计很糟糕，导致了这种混乱。

Answer 5

您的时区很可能落后于GMT（例如，GMT-5），因此您的时区中的10,000,000毫秒来自1969年12月31日，但由于java.util.Calendar中的月份为零，因此您的日历 - >文本转换存在缺陷你得到1969-11-31而不是预期的1969-12-31。

Answer 6

You can figure out yourself if you change your first c.setTimeInMillis(l); in c.clear();