Display warning message before 5 minutes of specified time using C# and VB.Net in ASP.Net

Hi merictokatlio...,

You need to use Ajax timer controlwhich will check and display the timer.

Refer below sample code and change as per your need.

HTML

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:Label ID="lblMessage" runat="server" />
        <asp:Timer ID="Timer1" runat="server" OnTick="TimerTick" Interval="1000" />
    </ContentTemplate>
</asp:UpdatePanel>

Code

C#

private DataTable GetData()
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[3] {
        new DataColumn("Id"), new DataColumn("Lesson"), new DataColumn("StartTime") });
    dt.Rows.Add(1, "Matematik", "16:45:00");

    return dt;
}

protected void TimerTick(object sender, EventArgs e)
{
    lblMessage.Text = "";
    DataTable dt = GetData();
    DateTime currentDateTime = DateTime.Now;
    foreach (DataRow dr in dt.Rows)
    {
        DateTime startTime = Convert.ToDateTime(dr["StartTime"]);
        double diff = startTime.Subtract(currentDateTime).TotalSeconds;
        if (diff < 300 && diff > 60)
        {
            lblMessage.Text = string.Format("{0} lession to be start in {1} minutes.", 
                                            dr["Lesson"].ToString(), Math.Round(diff / 60, 0));
        }
        else if (diff < 60 && diff > 0)
        {
            lblMessage.Text = string.Format("{0} lession to be start in {1} seconds.", 
                                            dr["Lesson"].ToString(), Math.Round(diff));
        }
        else if (diff == 0)
        {
            lblMessage.Text = string.Format("{0} lession started.", dr["Lesson"].ToString());
        }
    }
}

VB.Net

Private Function GetData() As DataTable
    Dim dt As DataTable = New DataTable()
    dt.Columns.AddRange(New DataColumn(2) {
                        New DataColumn("Id"), New DataColumn("Lesson"), New DataColumn("StartTime")})
    dt.Rows.Add(1, "Matematik", "16:45:00")
    Return dt
End Function

Protected Sub TimerTick(ByVal sender As Object, ByVal e As EventArgs)
    lblMessage.Text = ""
    Dim dt As DataTable = GetData()
    Dim currentDateTime As DateTime = DateTime.Now
    For Each dr As DataRow In dt.Rows
        Dim startTime As DateTime = Convert.ToDateTime(dr("StartTime"))
        Dim diff As Double = startTime.Subtract(currentDateTime).TotalSeconds

        If diff < 300 AndAlso diff > 60 Then
            lblMessage.Text = String.Format("{0} lession to be start in {1} minutes.",
                                            dr("Lesson").ToString(), Math.Round(diff / 60, 0))
        ElseIf diff < 60 AndAlso diff > 0 Then
            lblMessage.Text = String.Format("{0} lession to be start in {1} seconds.",
                                            dr("Lesson").ToString(), Math.Round(diff))
        ElseIf diff = 0 Then
            lblMessage.Text = String.Format("{0} lession started.", dr("Lesson").ToString())
        End If
    Next
End Sub

Note: Get the GetData data from database.