if (getComponent('comboBox1').getValue()==0)
    {'both'}

else
    {'date'}

<xp:comboBox id="comboBox1">
    <xp:selectItem itemLabel="One" itemValue="1"></xp:selectItem>
    <xp:selectItem itemLabel="Null" itemValue="0"></xp:selectItem>
       <xp:eventHandler event="onchange" submit="true" refreshMode="partial" refreshId="inputText1">
          <xp:this.action><![CDATA[#{javascript:
             var cmp:com.ibm.xsp.component.xp.XspInputText = getComponent("inputText1");
             var converter:com.ibm.xsp.convert.DateTimeConverter = cmp.getConverter();
             var value = getComponent("comboBox1").getValue();

             if( value == 0) {
                 converter.setType("both");
             }else{
                 converter.setType("date");
             }
          }]]>
       </xp:this.action>
    </xp:eventHandler>
</xp:comboBox>

<xp:inputText id="inputText1">
   <xp:this.converter>
      <xp:convertDateTime type="time" />
   </xp:this.converter>
   <xp:dateTimeHelper />
</xp:inputText>