openproject/public/templates/timelines/timeline_column.html

<span class="tl-historical" ng-show="rowObject.does_historical_differ(columnName)">
  <a href="javascript://" title="{{I18n.t('js.timelines.change')}}"
     ng-class="[
       'icon',
       'tl-icon-' + isDateColumn && historicalDateKind || 'tl-icon-changed'
     ]">
    {{rowObject.historical()[columnName] || I18n.t('js.timelines.empty')}}
  </a>
  <br/>
</span>

<span ng-class="[
  'tl-column',
  isDateColumn && 'tl-current',
  isDateColumn && rowObject.does_historical_differ(columnName) && 'tl-' + (historicalDateKind)
]">
  {{columnData}}
</span>