android - System.currentTimeMillis（）无法按预期工作

Question

I use System.currentTimeMillis() to save the time a user starts an activity.

public class TimeStamp {
protected long _startTimeMillis = System.currentTimeMillis();

public String getStartTime() {
    return new Time(_startTimeMillis).toString();
}

the class is instantiated when activity is started and getStartTime() returns the correct time. I also try to get the time that has passed after the activity has been started.

public String getElapsedTime() {
    return new Time(System.currentTimeMillis() - _startTimeMillis).toString();
}

This works perfectly fine using an emulated android device (android 4.0.3). But when I deploy the application on my real android device (android 4.0.3) getElapsedTime() starts with one additional hour and then counts up normally. So on my real device getElapsedTime() will return "01:00:01" after the activity was started, but it should return "00:00:01".

Do you have any ideas why this happens?

Answer 1

You should use SystemClock.uptimeMillis() .

uptimeMillis() is counted in milliseconds since the system was booted and is guaranteed to be monotonic whereas System.currentTimeMillis() isn't because the user or an application can change it at any time. Also, usually automatic time synchronization is enabled which can change the time at any time.

Answer 2

You can't use Time to represent a difference between 2 instants. In your code, getElapsedTime() returns a time close to 0, which is interpreted as 1970, January 1st. I think that the reason why you have a problem is because you don't have the same time zone on your device, hence the origin of time is not midnight.

Answer 3

System.currentTimeMillis() returns milliseconds passed between the device's clock time and the epoch date. That's why it can vary from device to device.

http://developer.android.com/reference/java/lang/System.html#currentTimeMillis ()