ASP.NET设置发帖时间间隔不超过30秒

正在做一个站，有论坛功能，为了避免发帖机或者恶意灌水，需要这样的限制。

protected void btnPost_Click(object sender, EventArgs e)
{
    if (Session["lastTime"] == null)
    {
        Session["lastTime"] = DateTime.Now;
    }
    DateTime dt = Convert.ToDateTime(Session["lastTime"]);
    TimeSpan ts = DateTime.Now - dt;
    Response.Write(ts.Days + ":" + ts.Hours + ":" + ts.Minutes + ":" + ts.Seconds);
    if (ts.Hours > 0 || ts.Minutes > 0 || ts.Seconds > 30)
    {
        Session["lastTime"] = DateTime.Now;
        Response.Write("可以发帖!");
    }
    else
    {
        Response.Write("你的发帖速度过快!时间间隔不要小于30秒！");
    }
}
/// <summary>
/// 把秒转换成分钟
/// </summary>
/// <returns></returns>
public static int SecondToMinute(int Second)
{
    decimal mm = (decimal)((decimal)Second / (decimal)60);
    return Convert.ToInt32(Math.Ceiling(mm));
}
/// <summary>
/// 计算两个时间的时间间隔
/// </summary>
/// <param name="DateTimeOld">较早的日期和时间</param>
/// <param name="DateTimeNew">较后的日期和时间</param>
/// <returns></returns>
public static string DateDiff(DateTime DateTimeOld, DateTime DateTimeNew)
{
    string dateDiff = "";
    TimeSpan ts1 = new TimeSpan(DateTimeOld.Ticks);
    TimeSpan ts2 = new TimeSpan(DateTimeNew.Ticks);
    TimeSpan ts = ts1.Subtract(ts2).Duration();
    int day = ts.Days;
    int hou = ts.Hours;
    int minu = ts.Minutes;
    int sec = ts.Seconds;
    if (day > 0)
    {
        if (day > 30)
        {
            if (day > 364)
            {
                dateDiff += day / 365 + "年";
            }
            else
            {
                dateDiff += day / 30 + "个月";
            }
        }
        else
        {
            dateDiff += day.ToString() + "天";
        }
    }
    else
    {
        if (hou > 0)
        {
            dateDiff += hou.ToString() + "小时";
        }
        else
        {
            if (minu > 0)
            {
                dateDiff += minu.ToString() + "分钟";
            }
            else
            {
                if (sec > 0)
                {
                    dateDiff += sec.ToString() + "秒";
                }
                else
                {
                    dateDiff += "0秒";
                }
            }
        }
    }
    if (DateTimeNew.CompareTo(DateTimeOld) > 0)
    {
        dateDiff += "前";
    }
    else
    {
        dateDiff += "后";
    }
    return dateDiff;
}
/// <summary>
///判断是否于5分钟之前
/// </summary>
/// <param name="DateTimeOld">较早的日期和时间</param>
/// <returns></returns>
public static bool DateDiff_minu(DateTime DateTimeOld)
{
    TimeSpan ts1 = new TimeSpan(DateTimeOld.Ticks);
    TimeSpan ts2 = new TimeSpan(DateTime.Now.Ticks);
    TimeSpan ts = ts1.Subtract(ts2).Duration();
    int minu = ts.Minutes;
    if (minu > 5)
    {
        return true;
    }
    else
    {
        return false;
    }
}
/// <summary>
/// 与当前时间比较,重载时间比较函数,只有一个参数
/// </summary>
/// <param name="DateTimeOld">较早的日期和时间</param>
/// <returns></returns>
public static string DateDiff(DateTime DateTimeOld)
{
    return DateDiff(DateTimeOld, DateTime.Now);
}
/// <summary>
/// 日期比较,返回精确的几分几秒
/// </summary>
/// <param name="DateTime1">较早的日期和时间</param>
/// <param name="DateTime2">较迟的日期和时间</param>
/// <returns></returns>
public static string DateDiff_full(DateTime DateTime1, DateTime DateTime2)
{
    string dateDiff = null;
    TimeSpan ts1 = new TimeSpan(DateTime1.Ticks);
    TimeSpan ts2 = new TimeSpan(DateTime2.Ticks);
    TimeSpan ts = ts1.Subtract(ts2).Duration();
    dateDiff = ts.Days.ToString() + "天" + ts.Hours.ToString() + "小时" + ts.Minutes.ToString() + "分钟" + ts.Seconds.ToString() + "秒";
    return dateDiff;
}
/// <summary>
/// 时间比较,返回精确的几秒
/// </summary>
/// <param name="DateTime1">较早的日期和时间</param>
/// <param name="DateTime2">较迟的日期和时间</param>
/// <returns></returns>
public static int DateDiff_Sec(DateTime DateTime1, DateTime DateTime2)
{
    TimeSpan ts1 = new TimeSpan(DateTime1.Ticks);
    TimeSpan ts2 = new TimeSpan(DateTime2.Ticks);
    TimeSpan ts = ts1.Subtract(ts2).Duration();
    int dateDiff = ts.Days * 86400 + ts.Hours * 3600 + ts.Minutes * 60 + ts.Seconds;
    return dateDiff;
}


/// <summary>
/// 日期比较
/// </summary>
/// <param name="today">当前日期</param>
/// <param name="writeDate">输入日期</param>
/// <param name="n">比较天数</param>
/// <returns>大于天数返回true，小于返回false</returns>
public static bool CompareDate(string today, string writeDate, int n)
{
    DateTime Today = Convert.ToDateTime(today);
    DateTime WriteDate = Convert.ToDateTime(writeDate);
    WriteDate = WriteDate.AddDays(n);
    if (Today >= WriteDate)
        return false;
    else
        return true;
}