I would like to show the month in which the article was issued if and only if we also include the day.

Usually, I would go with something like <if variable="issued">. The problem is that I want to check just the date-part "day", not the whole "issued" variable.

But it is not possible to use the following:
        <date variable="issued">
          <choose>
            <if date-part="day">
              <date-part name="day" suffix=". "/>
              <date-part name="month" suffix=". " form="numeric"/>
            </if>
          </choose>
          <date-part name="year"/>
        </date>

And this does not work either:
        <choose>
          <if date-part="day">
            <date variable="issued">
              <date-part name="day" suffix=". "/>
              <date-part name="month" suffix=". " form="numeric"/>
              <date-part name="year"/>
            </date>
          </if>
          <else>
            <date variable="issued">
              <date-part name="year"/>
            </date>
          </else>
        </choose>


What is the proper way to check if the date-part "day" is not null?