Для разных версий необходимо по-разному сетить время для AlarmManager.
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
final AlarmManager.AlarmClockInfo alarmClockInfo = new AlarmManager.AlarmClockInfo(triggerTime, pendingIntent);
alarmManager.setAlarmClock(alarmClockInfo, pendingIntent);
}
else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
alarmManager.setExact(AlarmManager.RTC_WAKEUP, triggerTime, pendingIntent);
} else {
alarmManager.set(AlarmManager.RTC_WAKEUP, triggerTime, pendingIntent);
}