自制闹钟，响铃时间总是比设置时间晚3分钟左右？咋回事？

再简单不过的练手项目给我整郁闷了，现在的问题时App可以运行，假如我设置闹铃时间为9:00，那App到了9:00它不响铃，偏偏到了9:03才响铃，这是咋回事？我用的真机测试的，系统时间保证没有错误，时区设置也没有错误。

感谢各位前辈指点

下面是全部代码：
MainActivity.java

public class MainActivity extends Activity {


	private TimePicker timePicker;

	private TextView tv_time;

	private Button btn_setClock;

	private AlarmManager alarmManager;

	int hour;

	int min;


	@Override

	protected void onCreate(Bundle savedInstanceState) {

		super.onCreate(savedInstanceState);

		setContentView(R.layout.activity_main);


		timePicker = (TimePicker) findViewById(R.id.timePicker);

		tv_time = (TextView) findViewById(R.id.tv_time);

		btn_setClock = (Button) findViewById(R.id.btn_setClock);

		alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);


		timePicker.setOnTimeChangedListener(new OnTimeChangedListener() {

			@Override

			public void onTimeChanged(TimePicker view, int hourOfDay, int minute) {

				hour = hourOfDay;

				min = minute;

				tv_time.setText(hour + " : " + min);

			}

		});


		btn_setClock.setOnClickListener(new OnClickListener() {

			@Override

			public void onClick(View v) {

				Calendar calendar = Calendar.getInstance();

				calendar.setTimeInMillis(System.currentTimeMillis());

				calendar.set(Calendar.HOUR, hour);

				calendar.set(Calendar.MINUTE, min);

				calendar.set(Calendar.SECOND, 0);

				calendar.set(Calendar.MILLISECOND, 0);

				Intent intent = new Intent(MainActivity.this,

						AlarmReceiver.class);

				PendingIntent pendingIntent = PendingIntent.getBroadcast(

						MainActivity.this, 0, intent, 0);

				alarmManager.set(AlarmManager.RTC_WAKEUP,

						calendar.getTimeInMillis(), pendingIntent);

				Toast.makeText(MainActivity.this, "ok", 1000).show();

			}

		});

	}

}

AlarmReceiver.java

public class AlarmReceiver extends BroadcastReceiver {

	@Override

	public void onReceive(Context context, Intent intent) {

		Log.d("Debug", "onReceive");

		Intent i = new Intent(context, AlarmActivity.class);

		i.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

		context.startActivity(i);

	}

}

AlarmActivity.java

public class AlarmActivity extends Activity {

	@Override

	protected void onCreate(Bundle savedInstanceState) {

		super.onCreate(savedInstanceState);

		AlertDialog.Builder alertDialog = new AlertDialog.Builder(

				AlarmActivity.this);

		alertDialog.setTitle("闹钟");

		alertDialog.setMessage("时间到了！");

		alertDialog.setCancelable(false);

		alertDialog.setPositiveButton("OK",

				new DialogInterface.OnClickListener() {

					@Override

					public void onClick(DialogInterface dialog, int which) {

						AlarmActivity.this.finish();

					}

				});

		alertDialog.show();

	}

}

AndroidManifest.xml

        ...

        

        

        

        
            android:name=".AlarmReceiver"

            android:process=":remote" />