原来写的用户控件，但是维护太麻烦，后来就写了一个自定义控件，比较好维护，使用也方便多了。
该控件是一个时间控件，通过下拉列表框选择时间的年、月、日、时、分、秒。初始化时要提供可选时间的
起始年份。

源代码如下：

using System;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.ComponentModel;
using System.ComponentModel.Design;

namespace UserTime
{
/// <summary>
/// 用户自定义时间控件，取到年、月、日、时、分、秒
/// </summary>
[DefaultProperty("Text"),
ToolboxData("<{0}:clsUserTime runat=server></{0}:clsUserTime>")]
//INamingContainer为子控件提供了一个新的命名范围,保证子控件的ID唯一性
public class clsUserTime : System.Web.UI.WebControls.WebControl, INamingContainer
{
private System.Web.UI.WebControls.DropDownList ddlYear;
private DropDownList ddlMonth;
private DropDownList ddlDay;
private DropDownList ddlHour;
private DropDownList ddlMinite;
private DropDownList ddlSecond;

private static int beginYear = 0; //起始年号，这里必须使用static以防二次调用后变量被清零
private static bool IsInit = false; //是否已经初始化过，这里必须使用static以防二次调用后变量被清零

public int Year //取控件时间：年
{
get
{
return ddlYear.SelectedIndex + beginYear;
}
}

public int Month //取控件时间：月
{
get
{
return ddlMonth.SelectedIndex + 1;
}
}

public int Day //取控件时间：日
{
get
{
return ddlDay.SelectedIndex + 1;
}
}

public int Hour //取控件时间：时
{
get
{
return ddlHour.SelectedIndex;
}
}

public int Minite //取控件时间：分
{
get
{
return ddlMinite.SelectedIndex;
}
}

public int Second //取控件时间：秒
{
get
{
return ddlSecond.SelectedIndex;
}
}

//用起始年号初始化控件，如果已经初始化过控件，直接返回
public void InitControls(int startYear, int endYear)
{
if (IsInit)
{
return;
}
EnsureChildControls();
if (startYear > endYear)
{
int tmp = startYear;
startYear = endYear;
endYear = startYear;
}
beginYear = startYear;
for (int i = startYear; i <= endYear; ++i)
{
ddlYear.Items.Add(i.ToString());
}
for (int i = 1; i <=12; ++i)
{
ddlMonth.Items.Add(i.ToString());
}
for (int i = 1; i <= 31; ++i)
{
ddlDay.Items.Add(i.ToString());
}
for (int i = 0; i < 24; ++i)
{
ddlHour.Items.Add(i.ToString());
}
for (int i = 0; i < 60; ++i)
{
ddlMinite.Items.Add(i.ToString());
ddlSecond.Items.Add(i.ToString());
}
IsInit = true;
}
//重载Controls属性，取得ControlCollection对象，该对象表示UI层次结构中的服务器控件的子控件
public override ControlCollection Controls
{
get
{
//确定服务器是否包含子控件
EnsureChildControls();
return base.Controls;
}
}

//动态创建子控件
protected override void CreateChildControls()
{
//创建年、月、日、时、分、秒下拉列表框
this.Controls.Clear();
ddlYear = new DropDownList();
ddlYear.ID = "ddlYear";
ddlYear.Width = 54;

ddlMonth = new DropDownList();
ddlMonth.ID = "ddlMonth";
ddlMonth.Width = 40;

ddlDay = new DropDownList();
ddlDay.ID = "ddlDay";
ddlDay.Width = 40;

ddlHour = new DropDownList();
ddlHour.ID = "ddlHour";
ddlHour.Width = 40;

ddlMinite = new DropDownList();
ddlMinite.ID = "ddlMinite";
ddlMinite.Width = 40;

ddlSecond = new DropDownList();
ddlSecond.ID = "ddlSecond";
ddlSecond.Width = 40;

this.Controls.Add(ddlYear);
this.Controls.Add(ddlMonth);
this.Controls.Add(ddlDay);
this.Controls.Add(dd