Here's how to display the last modification date of a wiki in a banner in the corner of the window:

1. Install the plugin Corner ribbon in your TiddlyWiki
2. Save () and reload () your wiki
3. Create a new tiddler called $:/_MyRibbon tagged $:/tags/PageControls and containing:

   <div class="github-fork-ribbon-wrapper right">
   <div class="github-fork-ribbon" style="background-color:#DF4848;">
   <$list filter="[!is[system]!has[draft.of]!sort[modified]limit[1]]">
   <$link>
   <$view field="modified" format="date" template="DD mmm YYYY at 0hh:0mm"/>
   </$link>
   </$list>
   </div>
   </div>

4. If required, change the background-color value to your preference
5. You can also try changing the positioning class from right to right-bottom
   1. To make the banner appear at the top left you'll need to tag the tiddler $:/tags/PageTemplate instead of $:/tags/PageControls and then change the position class to left