java - 将系统时间更改为提前三个小时，然后恢复为默认值

Question

我一直在尝试将系统时间更改为提前三个小时，然后恢复为默认值，但我似乎没有得到我想要的时间。不是 cal.setTimeZone(TimeZone.getTimeZone("America/Los_Angeles"));是否要将时间设置为 America/Los_Angeles 时区？

public static void changeSystemTime() throws Exception {

    DateFormat dateFormat = new SimpleDateFormat("HH:mm");

    Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("America/Los_Angeles"));

    System.out.println("Current time is " + dateFormat.format(cal.getTime());

    cal.add(Calendar.HOUR_OF_DAY, 3);
    System.out.println("3 Hours from now is " + dateFormat.format(cal.getTime());

    Runtime runtime = Runtime.getRuntime();

    runtime.exec("cmd /c Time " + dateFormat.format(cal.getTime()));
}

public static void changeSystemTimeToDefault() throws Exception {

    DateFormat dateFormat = new SimpleDateFormat("HH:mm");

    Date date = new Date();

    Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("America/Los_Angeles"));

    System.out.println("Timezone for LA is " + dateFormat.format(cal.getTime());

    Runtime runtime = Runtime.getRuntime();

    runtime.exec("cmd /c Time " + dateFormat.format(cal.getTime()));
}

Answer 1

你实际上没有在方法changeSystemTime()中添加3个小时；

原因:

String time = dateFormat.format(cal.getTime());
System.out.println("Current time is " + time);

cal.add(Calendar.HOUR_OF_DAY, 3);
// here you again work with the initial time, the 3 hours are not added to it
System.out.println("3 Hours from now is " + time);

这样做:

cal.add(Calendar.HOUR_OF_DAY, 3);
// get new time
System.out.println("3 Hours from now is " + cal.getTime());