I would like to get the installation-date of my joomla-installation and/or of any component by writing a new method in a model of my component.
How could I solve this?
On Linux the best you can do is work out the last modified time of a certain file by using filetime (http://php.net/filemtime). So to work out the Joomla installation date you could run filetime on a file that is not likely to have been edited /components/index.html.
You could use the same principle for extensions by doing :