Action取不到<sx:datetimepicker>标签的值

jsp页面用到sx:datetimepicker标签,action取不到日期值。
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addMaterial.jsp

 <%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<%@ taglib prefix="s" uri="/struts-tags" %>
<%@ taglib prefix="sx" uri="/struts-dojo-tags" %>

<html>

    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>addMaterial.jsp</title>
        <sx:head parseContent="true" extraLocales="UTF-8"/>
    </head>

    <body>
        <!-- 材料输入表单 -->
        <table>
            <s:form id="materialForm" action="addMaterial">
                <s:textfield name="material.material" label="材料名"></s:textfield>
                <s:textfield name="material.bid" label="价格"></s:textfield>
                <s:textfield name="material.mount" label="库存量"></s:textfield>
                <s:date name=""/>
                <sx:datetimepicker label="过期日期" name="material.expireDate" value="today" toggleType="plain" toggleDuration="300" language="utf-8" type="date" 
                    displayWeeks="5" displayFormat="dd/MM/yyyy" formatLength="long" />
                <s:submit value="提交"></s:submit>
            </s:form>
        </table>
    </body>

</html>

AddMaterialAction.java

 package action;

import com.opensymphony.xwork2.ActionSupport;

import model.Material;

public class AddMaterialAction extends ActionSupport {

    /**
     * 
     */
    private static final long serialVersionUID = 1L;

    // 属性类型需要类型转换的对象
    private Material material;

    public Material getMaterial() {
        return material;
    }

    public void setMaterial(Material material) {
        this.material = material;
    }

    public String execute() throws Exception {
        /*
         * SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd"); String
         * f=sdf.format(abc); System.out.println(f);
         */
        return SUCCESS;
    }
}

Material.java

 package model;

import java.util.Date;

public class Material {

    private int materialId;
    private String material;
    private double bid;
    private int mount;
    private Date expireDate;

    public Material() {
        super();
    }

    public int getMaterialId() {
        return materialId;
    }

    public void setMaterialId(int materialId) {
        this.materialId = materialId;
    }

    public String getMaterial() {
        return material;
    }

    public void setMaterial(String material) {
        this.material = material;
    }

    public double getBid() {
        return bid;
    }

    public void setBid(double bid) {
        this.bid = bid;
    }

    public int getMount() {
        return mount;
    }

    public void setMount(int mount) {
        this.mount = mount;
    }

    public Date getExpireDate() {
        return expireDate;
    }

    public void setExpireDate(Date expireDate) {
        this.expireDate = expireDate;
    }

}

把jsp页面的

标签去掉后台就能得到datetimepicker的值,不知道为什么,有
标签的时候其他值能传过去,就datetimepicker的值为null。