openproject/frontend/app/templates/timelines/timeline_column_data.html

<span class="tl-historical" ng-if="historicalDataDiffers">
  <span ng-bind="historicalData" class="historical-data"></span>
  <a href="javascript://" title="{{labelTimelineChanged}}"
     ng-class="[
       'icon',
       'tl-icon-' + (isDateColumn && historicalDateKind || 'changed'),
       'icon-' + (isDateColumn && 'time' || 'arrow-left-right')
     ]"></a>
  <br/>
</span>

<span ng-class="[
  'tl-column',
  isDateColumn && 'tl-current',
  isDateColumn && historicalDataDiffers && 'tl-' + historicalDateKind
]" ng-bind="columnData"></span>