判断年月日的方法

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    protected string getChangeDay(string month,int  year)
    {
        if ((month == "01") || (month == "03") || (month == "05") || (month == "07") || (month == "08") || (month == "10") || (month == "12"))
        {
            return "31";
        }
        else if (month == "02")
        {
            if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
            {
                return "28";
            }
            else { return "29"; }
        }
        else
        {
            return "30";
        }
    }
    protected void ddlYear_SelectedIndexChanged(object sender, EventArgs e)
    {
        getChage();
    }

    private void getChage()
    { 
        txtCreator.Value = string.Empty;
        string year = ddlYear.SelectedValue;
        string month = ddlMonth.SelectedValue;
        if (!string.IsNullOrEmpty(year))
        {
            txtStartDate.Text = year + "-" + month + "-" + "01";
            txtEndDate.Text = year + "-" + month + "-" + getChangeDay(month, Convert.ToInt32(year));
        }       
    }
    protected void ddlMonth_SelectedIndexChanged(object sender, EventArgs e)
    {
        getChage();
    }