ui.bootstrap.datepicker is-open not working in modal

Just change to: is-open="opened" to:

is-open="$parent.opened"

Fixed Demo Plunker

So relevant snippets of HTML will look like:

      <div class="input-group">

          <input type="text" class="form-control" 
                 datepicker-popup="dd.MM.yyyy"
                 ng-model="dt"
                 is-open="$parent.opened"
                 ng-required="true"
                 close-text="Close" />
          <span class="input-group-btn">
          <button style="height:34px;" class="btn btn-default" ng-click="open()">
          <i class="icon-calendar"></i></button> <b><- button not working</b>
          </span>
        </div>