Hello, özer,
According to the provided information, it is not clear enough what is the Telerik product that you are currently using since this is the forum related to the Telerik UI for WinForms suite but you mentioned "web page". I am providing an answer regarding the Telerik WinForms product. In case you are using another product, feel free to post your question in the relevant forum: https://www.telerik.com/forums
The provided information is also not enough to get full understanding of the precise case and the setup that you have. However, I will try to suggest a possible solution.
If you have an assigned value to a cell in GridViewDateTimeColumn, the cell is expected to show this value. Note that a GridViewDateTimeColumn offers the NullValue property. Thus, you specify what date value to be displayed within the cell if you have DBNull.Value or null.
GridViewDateTimeColumn col1 = new GridViewDateTimeColumn("Date1");
col1.NullValue = new DateTime(2020, 1, 1);
this.radGridView1.Columns.Add(col1);
GridViewDateTimeColumn col2 = new GridViewDateTimeColumn("Date2");
this.radGridView1.Columns.Add(col2);
this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
this.radGridView1.Rows.Add(DateTime.Now, DateTime.Now);
this.radGridView1.Rows.Add(null, null);
this.radGridView1.Rows.Add(DBNull.Value, DBNull.Value);
this.radGridView1.Rows.Add(DateTime.Now.AddDays(-1), DateTime.Now.AddDays(-1));

If it is not the case or it doesn't fit your scenario, it would be greatly appreciated if you can provide more details about the exact goal that you are trying to achieve. Once we get better understanding of the precise case, we can about a suitable solution and provide further assistance. Thank you in advance for your cooperation.
I hope this information helps. If you need any further assistance please don't hesitate to contact me.
Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik
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