How can i make ColumnHeaderName to HH:mm format?

string columnHeaderName = dgvSchedule.Columns[i].HeaderText.ToString();

       DateTime timeColumn = Convert.ToDateTime(columnHeaderName);

You want that the ColumnHeader of one column in the DGV shows the time? and the time in thr real time?

yes

Simple :)
You will have to use a Timer object to update the time in the DGV`s headerText:

System.Windows.Forms.Timer timer1;
        public Form1()
        {
            InitializeComponent();
            dataGridView1.Columns.Add("col1", "");
            dataGridView1.Columns.Add("col2", "Column 2");
            dataGridView1.Columns[0].HeaderCell.Style.Alignment = DataGridViewContentAlignment.MiddleCenter;
            ShowTime();

            timer1 = new Timer();
            timer1.Interval = 1000;
            timer1.Tick += new EventHandler(timer1_Tick);
            timer1.Start();
        }        

        private void timer1_Tick(object sender, EventArgs e)
        {
            ShowTime();
        }

        private void ShowTime()
        {
            string time = String.Format("{0:HH:mm:ss}", DateTime.Now);
            dataGridView1.Columns[0].HeaderText = time;
        }
static void Main(string[] args)
        {

            DateTime dt = DateTime.Now;

            string Dstr = String.Format("{0:t}", dt);
            Console.WriteLine(Dstr);
            // This is the same:
            Console.WriteLine("{0:t}", dt);
 
            Console.ReadKey();
}
static void Main(string[] args)
        {

            DateTime dt = DateTime.Now;

            string Dstr = String.Format("{0:t}", dt);
            Console.WriteLine(Dstr);
            // This is the same:
            Console.WriteLine("{0:t}", dt);
 
            Console.ReadKey();
}

its not that i want to convert from datetime to string.. I want to convert my string to datetime.. and the datetime has a format of HH:mm

have you checked my solution?

yes i did i did not try it as i already entered the values in the datagridview frm the design

You better try it, you will see it works.

btw, can I see your code?

yes oFcourse.. ill try it l8er so i will do more things

public void FillSechedule()
        {
            dgvSchedule.TopLeftHeaderCell.Value = "Time\nTable";

            ManagerTables mgrTables = new ManagerTables();
            List<Table> listOfAllTables = mgrTables.GetAllTables();

            ManagerTableReservation mgrTableReservation = new ManagerTableReservation();
            ManagerReservations mgrReservation = new ManagerReservations();

            foreach(Table t in listOfAllTables) 
            {
                int n = dgvSchedule.Rows.Add();
                dgvSchedule.Rows[n].HeaderCell.Value = t.TableID.ToString(); //"Table " + t.TableID.ToString();
            }

            for (int i = 0; i < listOfAllTables.Count; i++) //used to loop all tables
            {
                string rowHeaderName = dgvSchedule.Rows[i].HeaderCell.Value.ToString();
                int tableNo = Convert.ToInt32(rowHeaderName);

                List<TableReservation> listOfTableReservation = mgrTableReservation.GetReservationsByTableID(tableNo);

                foreach (TableReservation tr in listOfTableReservation)//used to llop through all table reservations
                {
                    Reservation res = mgrReservation.GetReservationsByReservationID(tr.ReservationID);

                    DateTime dateSelected = Convert.ToDateTime(dtpDateTimeSchedule.Value.ToString("dd/MM/yyyy"));
                    DateTime resDate = Convert.ToDateTime(res.DateTimeOfEvent.ToShortDateString());

                    if (dateSelected==resDate)//(res.DateTimeOfEvent.ToString("MM/dd/yyyy") == dateSelected.ToString())
                    {
                        for (int a = 0; a < dgvSchedule.Rows.Count; a++)//Loop thorough rows
                        {

                            Table t = listOfAllTables[i];

                            if (t.TableID == tableNo)
                            {
                                for (int b = 0; b < dgvSchedule.Columns.Count; b++)//Loop thorough columns
                                {
                                    string columnHeaderName = dgvSchedule.Columns[i].HeaderText.ToString();
                                    

                                    DateTime timeColumn = Convert.ToDateTime(String.Format("{0:t}", columnHeaderName));
                                    DateTime dateTimeOfReservation = Convert.ToDateTime(res.DateTimeOfEvent.ToShortTimeString());
                                    DateTime dateTimeOfReservationPlus = Convert.ToDateTime(res.DateTimeOfEvent.AddMinutes(30).ToShortTimeString());

                                    
                                    if (dateTimeOfReservation >= timeColumn && dateTimeOfReservationPlus <= timeColumn && res.Cancelled.Equals(false))
                                    {
                                        dgvSchedule.Rows[a].Cells[b].Style.BackColor = Color.Chartreuse;//.color
                                        dgvSchedule.Rows[a].Cells[b].Value = res.CustomerID.ToString();
                                    }
                                    else
                                    {
                                        continue;
                                    }
                                }
                            }
                            else
                            {
                                continue;
                            }
                        }
                    }
                    else
                    {
                        continue;
                    }
                    //Foreach ohra bix igib column column tad dgv
                    //loop all reservations and check if time is == torow time, If yes highlight cell.. else continue
                }

            }
        }
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