The Selfoss Android client by yDelouis expects time to be in UTC. As I am using UTC +2 or UTC +1 on my server it will always show the wrong time.
The problem is already in the issues, but the discussion there is from 2014 and is basically suggesting changing the server time to UTC.

As much as this feels like wagging the dog, it is my only chance, since I don't have a current Android IDE (and cannot code Java anyway).

Trying to solve this on the server side, I put

date_default_timezone_set('UTC');

just after the namespace import in these files:

helpers/ContentLoader.php
helpers/Viewhelper.php

So far this works for me. datetime in selfoss_items on the the server (MySQL) will be UTC. Android client and web show the correct time.
I don't know what it might break, but feed updates seem to work fine.

PHP 5.5.38
MySQL 5.7.15