This is the code I'm using to insert all the details into a google calendar app popup.
Everything is working (right date, title, description) except if you click on "save" on the calendar popup it says: event saved successfully but the event doesn't actually get saved in the calendar.
addToCalendar.setOnClickListener(v -> {
Calendar beginTime = Calendar.getInstance();
beginTime.set(datumvonCalendar.getJahr(),datumvonCalendar.getMonat(),datumvonCalendar.getTag(),zeitvonCalendar.getStunde(),zeitvonCalendar.getMinute(),0);
Calendar endTime = Calendar.getInstance();
endTime.set(datumbisCalendar.getJahr(),datumbisCalendar.getMonat(),datumbisCalendar.getTag(),zeitbisCalendar.getStunde(),zeitbisCalendar.getMinute(),0);
Long millisEnd = endTime.getTimeInMillis();
Long millisStart = beginTime.getTimeInMillis();
Intent intent = new Intent(Intent.ACTION_INSERT);
intent.setData(CalendarContract.Events.CONTENT_URI);
intent.putExtra(CalendarContract.EXTRA_EVENT_BEGIN_TIME,millisStart);
intent.putExtra(CalendarContract.EXTRA_EVENT_END_TIME,millisEnd);
intent.putExtra(CalendarContract.Events.EVENT_TIMEZONE,"Europe/Berlin");
intent.putExtra(CalendarContract.Events.TITLE,headerTxt.getText().toString());
intent.putExtra(CalendarContract.Events.DESCRIPTION,beschreibungTxt.getText().toString());
fragment.startActivity(intent);
});