dayOfTheMonth.xml

<configuration doc:name="Configuration">
  <expression-language>
      <global-functions>
          def dayOfTheMonth(date){
          
              import java.util.Calendar;
              import java.text.SimpleDateFormat;
              
              cal = Calendar.getInstance();
              sdf = new SimpleDateFormat('yyyyMMdd');
              cal.setTime(sdf.parse(date));
              
              return cal.getActualMaximum(Calendar.DAY_OF_MONTH).toString();
          }
      </global-functions>
  </expression-language>
</configuration>