How to set a reminder in Android?

You can use AlarmManager in coop with notification mechanism Something like this:

Intent intent = new Intent(ctx, ReminderBroadcastReceiver.class);
PendingIntent pendingIntent = PendingIntent.getBroadcast(ctx, 0, intent, PendingIntent.FLAG_UPDATE_CURRENT);
AlarmManager am = (AlarmManager) ctx.getSystemService(Activity.ALARM_SERVICE);
// time of of next reminder. Unix time.
long timeMs =...
if (Build.VERSION.SDK_INT < 19) {
    am.set(AlarmManager.RTC_WAKEUP, timeMs, pendingIntent);
} else {
    am.setExact(AlarmManager.RTC_WAKEUP, timeMs, pendingIntent);
}

It starts alarm.

public class ReminderBroadcastReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        NotificationCompat.Builder builder = new NotificationCompat.Builder(context)
                .setSmallIcon(...)
                .setContentTitle(..)
                .setContentText(..);
        Intent intentToFire = new Intent(context, Activity.class);
        PendingIntent pendingIntent = PendingIntent.getActivity(context, 0, intentToFire, PendingIntent.FLAG_UPDATE_CURRENT);
        builder.setContentIntent(pendingIntent);
        NotificationManagerCompat.from(this);.notify((int) System.currentTimeMillis(), builder.build());
    }
}

Nope, it is more complicated than just calling a method, if you want to transparently add it into the user's calendar.

You've got a couple of choices;

1. Calling the intent to add an event on the calendar
   This will pop up the Calendar application and let the user add the event. You can pass some parameters to prepopulate fields:

   Calendar cal = Calendar.getInstance();              
   Intent intent = new Intent(Intent.ACTION_EDIT);
   intent.setType("vnd.android.cursor.item/event");
   intent.putExtra("beginTime", cal.getTimeInMillis());
   intent.putExtra("allDay", false);
   intent.putExtra("rrule", "FREQ=DAILY");
   intent.putExtra("endTime", cal.getTimeInMillis()+60*60*1000);
   intent.putExtra("title", "A Test Event from android app");
   startActivity(intent);
   

   Or the more complicated one:

2. Get a reference to the calendar with this method
   (It is highly recommended not to use this method, because it could break on newer Android versions):

   private String getCalendarUriBase(Activity act) {
   
       String calendarUriBase = null;
       Uri calendars = Uri.parse("content://calendar/calendars");
       Cursor managedCursor = null;
       try {
           managedCursor = act.managedQuery(calendars, null, null, null, null);
       } catch (Exception e) {
       }
       if (managedCursor != null) {
           calendarUriBase = "content://calendar/";
       } else {
           calendars = Uri.parse("content://com.android.calendar/calendars");
           try {
               managedCursor = act.managedQuery(calendars, null, null, null, null);
           } catch (Exception e) {
           }
           if (managedCursor != null) {
               calendarUriBase = "content://com.android.calendar/";
           }
       }
       return calendarUriBase;
   }
   

   and add an event and a reminder this way:

   // get calendar
   Calendar cal = Calendar.getInstance();     
   Uri EVENTS_URI = Uri.parse(getCalendarUriBase(this) + "events");
   ContentResolver cr = getContentResolver();
   
   // event insert
   ContentValues values = new ContentValues();
   values.put("calendar_id", 1);
   values.put("title", "Reminder Title");
   values.put("allDay", 0);
   values.put("dtstart", cal.getTimeInMillis() + 11*60*1000); // event starts at 11 minutes from now
   values.put("dtend", cal.getTimeInMillis()+60*60*1000); // ends 60 minutes from now
   values.put("description", "Reminder description");
   values.put("visibility", 0);
   values.put("hasAlarm", 1);
   Uri event = cr.insert(EVENTS_URI, values);
   
   // reminder insert
   Uri REMINDERS_URI = Uri.parse(getCalendarUriBase(this) + "reminders");
   values = new ContentValues();
   values.put( "event_id", Long.parseLong(event.getLastPathSegment()));
   values.put( "method", 1 );
   values.put( "minutes", 10 );
   cr.insert( REMINDERS_URI, values );
   

   You'll also need to add these permissions to your manifest for this method:

   <uses-permission android:name="android.permission.READ_CALENDAR" />
   <uses-permission android:name="android.permission.WRITE_CALENDAR" />
   

Update: ICS Issues

The above examples use the undocumented Calendar APIs, new public Calendar APIs have been released for ICS, so for this reason, to target new android versions you should use CalendarContract.

More infos about this can be found at this blog post.