apex:actionRegion

<!-- For this example to render fully, associate the page
with a valid opportunity record in the URL.
For example: https://MyDomain_login_URL/apex/myPage?id=001D000000IRt53 -->

<apex:page standardController="Opportunity">
  <apex:form >
    <apex:pageBlock title="Edit Opportunity" id="thePageBlock" mode="edit">

      <apex:pageBlockButtons >
        <apex:commandButton value="Save" action="{!save}"/>
        <apex:commandButton value="Cancel" action="{!cancel}"/>
      </apex:pageBlockButtons>

    <apex:pageBlockSection columns="1">
      <apex:inputField value="{!opportunity.name}"/>
      <apex:pageBlockSectionItem>
      <apex:outputLabel value="{!$ObjectType.opportunity.fields.stageName.label}"
                        for="stage"/>
      <!--
           Without the actionregion, selecting a stage from the picklist would cause
           a validation error if you hadn't already entered data in the required name
           and close date fields.  It would also update the timestamp.
      -->
      <apex:actionRegion>
        <apex:inputField value="{!opportunity.stageName}" id="stage">
          <apex:actionSupport event="onchange" rerender="thePageBlock"
                              status="status"/>
          </apex:inputField>
          </apex:actionRegion>
      </apex:pageBlockSectionItem>
        <apex:inputfield value="{!opportunity.closedate}"/>
        {!text(now())}
        </apex:pageBlockSection>

      </apex:pageBlock>
    </apex:form>
</apex:page>