Round up the time to the nearest 30 minutes using C# in ASP.Net

Hello.

I have meeting room booking system in which user can select meeting rooms. We have dropdown list for the timings as 8:00 AM, 8:30 Am, 9:00 AM … and so on. I want to show upcoming meeting like suppose the meeting room is booked at today 2017-08-28 10:00:00.000 and the current time is 2017-08-28 09:57 so no records is displayed. I want if the time is in between 08:00 – 08:29 it should covert time to 08:30 and if the time is between 08:31 – 08:59 it should convert to 09:00 and so on for the time till 05:00 PM.

If Time 08:00 – 08:29 then 08:30 else 08:31 – 08:59 then 09:00

 protected void Page_Load(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(connectionString);
        SqlCommand cmd = new SqlCommand("Select * from Meetings", con);
        SqlDataAdapter sda = new SqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        sda.Fill(dt);
        string date = dt.Rows[0]["meetingstarttime"].ToString();
        DateTime datesql = Convert.ToDateTime(date);

       
        DateTime s = DateTime.Now;
        TimeSpan ts = new TimeSpan(10, 30, 0);
        
      
         string currentdate = DateTime.Now.ToString("yyyy-MM-dd HH:mm");
         var newDate = DateTime.ParseExact(currentdate, "yyyy-MM-dd HH:mm", System.Globalization.CultureInfo.InvariantCulture);

         TimeSpan ts1 = new TimeSpan(00, 30, 0);
         s = newDate.Date + ts1;


        SqlCommand cmd1 = new SqlCommand("Select * from Meetings where meetingstarttime= '" + currentdate +"' ", con);
        SqlDataAdapter sda1 = new SqlDataAdapter(cmd1);
        DataTable dt1 = new DataTable();
        sda1.Fill(dt1);
       

        Label2.Text = dt.Rows[0]["meetingstarttime"].ToString();
        Label4.Text = dt.Rows[0]["meetingtitle"].ToString();
    }

<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" CausesValidation="True"
                    ValidationGroup="1" Width="145px" CssClass="form" 
                    onselectedindexchanged="DropDownList1_SelectedIndexChanged">
                    <asp:ListItem Value=" 12:00:00 AM">&lt;From Time&gt;</asp:ListItem>
                    <asp:ListItem Value=" 7:01:00 AM"> 7:00:00 AM</asp:ListItem>
                    <asp:ListItem Value=" 7:31:00 AM"> 7:30:00 AM</asp:ListItem>
                    <asp:ListItem Value=" 8:01:00 AM"> 8:00:00 AM</asp:ListItem>
                    <asp:ListItem Value=" 8:31:00 AM"> 8:30:00 AM</asp:ListItem>
                    <asp:ListItem Value=" 9:01:00 AM"> 9:00:00 AM</asp:ListItem>
                    <asp:ListItem Value=" 9:31:00 AM"> 9:30:00 AM</asp:ListItem>
                    <asp:ListItem Value=" 10:01:00 AM"> 10:00:00 AM</asp:ListItem>
                    <asp:ListItem Value=" 10:31:00 AM"> 10:30:00 AM</asp:ListItem>
                    <asp:ListItem Value=" 11:01:00 AM"> 11:00:00 AM</asp:ListItem>
                    <asp:ListItem Value=" 11:31:00 AM"> 11:30:00 AM</asp:ListItem>
                    <asp:ListItem Value=" 12:01:00 PM"> 12:00:00 PM</asp:ListItem>
                    <asp:ListItem Value=" 12:31:00 PM"> 12:30:00 PM</asp:ListItem>
                    <asp:ListItem Value=" 01:00:00 PM"> 01:00:00 PM</asp:ListItem>
                    <asp:ListItem Value=" 01:31:00 PM"> 01:30:00 PM</asp:ListItem>
                    <asp:ListItem Value=" 02:01:00 PM"> 02:00:00 PM</asp:ListItem>
                    <asp:ListItem Value=" 02:31:00 PM"> 02:30:00 PM</asp:ListItem>
                    <asp:ListItem Value=" 03:01:00 PM"> 03:00:00 PM</asp:ListItem>
                    <asp:ListItem Value=" 03:31:00 PM"> 03:30:00 PM</asp:ListItem>
                    <asp:ListItem Value=" 04:01:00 PM"> 04:00:00 PM</asp:ListItem>
                    <asp:ListItem Value=" 04:31:00 PM"> 04:30:00 PM</asp:ListItem>
                </asp:DropDownList>

Thanks