Android alarm is 12 hours late

Question

I have set an alarm (in an alarm clock app we are developing) for 8:58 (in 24h format).

The alarm did not fire - alarm icon is still in status bar - and when I write adb shell dumpsys alarm I can see this:

Batch{6b33bdb num=1 start=99803288 end=99803288 flgs=0x3}:
  RTC_WAKEUP #0: Alarm{3e3da2e type 0 when 1484078280736 my.app}
  tag=*walarm*:my.app/md5f98255d9820be5cc9672d0645bf1ca12.RingBroadcastReceiver
  type=0 whenElapsed=+11h1m53s125ms when=2017-01-10 20:58:00
  window=0 repeatInterval=0 count=0 flags=0x3
  Alarm clock:
    triggerTime=2017-01-10 08:58:00
    showIntent=PendingIntent{21219cf: PendingIntentRecord{aac3f5c my.app startActivity}}
  operation=PendingIntent{ecde88f: PendingIntentRecord{821c71c my.app broadcastIntent}}

Next wake from idle: Alarm{3e3da2e type 0 when 1484078280736 my.app}
  tag=*walarm*:my.app/md5f98255d9820be5cc9672d0645bf1ca12.RingBroadcastReceiver
  type=0 whenElapsed=+11h1m53s125ms when=2017-01-10 20:58:00
  window=0 repeatInterval=0 count=0 flags=0x3
  Alarm clock:
    triggerTime=2017-01-10 08:58:00
    showIntent=PendingIntent{21219cf: PendingIntentRecord{aac3f5c my.app startActivity}}
  operation=PendingIntent{ecde88f: PendingIntentRecord{821c71c my.app broadcastIntent}}

THE QUESTION IS: How is it possible that when is 12 hours after triggerTime?

It seems like 12/24 hour format issue, but my device and also my app and its time picker are all set to 24 hour format. This issue happened just once. When I set a new alarm, it works as expected.

Any ideas?

UPDATE: We are using this code to set the alarm (sorry for Xamarin syntax) and I have tested that the alarmEntity is returning correct time. The problem must be in this code or in the device/android/system stuff. From the triggerTime=2017-01-10 08:58:00 it seems to me that the code is OK and there is some issue in the system or alarm manager.

DateTime closestTrigger = alarmEntity.GetTime();

Calendar calendar = Calendar.Instance;
calendar.Set(CalendarField.Year, closestTrigger.Year);
calendar.Set(CalendarField.Month, closestTrigger.Month - 1);
calendar.Set(CalendarField.DayOfMonth, closestTrigger.Day);
calendar.Set(CalendarField.HourOfDay, closestTrigger.Hour);
calendar.Set(CalendarField.Minute, closestTrigger.Minute);
calendar.Set(CalendarField.Second, closestTrigger.Second);

var alarmInfo = new AlarmManager.AlarmClockInfo(calendar.TimeInMillis, mainPendingIntent);
manager.SetAlarmClock(alarmInfo, ringPendingIntent);

UPDATE 2: I've found one more important line in the same dumpsys alarm log:

Next alarm clock information: 
user:0 pendingSend:false time:1484035080736 = 2017-01-10 08:58:00 = -58m6s876ms

..which means the alarm was missed, but the system didn't even try to present it, the logcat didn't show any activity at all around the 8:58. And still not clear why would the system reschedule the alarm to 20:58.