The "DST Rule" label now looks like an HTML link (the text is blue and underlined), and clicking on it will open the window explaining the available DST rules.

This window was a KMessageBox::informationList(), but I'm now creating my own KDialog, because I wanted to be able to increase the width of the window so there's no need for a horizontal scrollbar. Apparently the KMessageBox can't resize itself to fit the contents.