To reproduce: use the following code snippet and enter in the filter row one of the row values as it is shown in the screenshot:

this.radGridView1.EnableFiltering = true;
GridViewDateTimeColumn dateTimeColumn = new GridViewDateTimeColumn("DateTimeColumn");  
dateTimeColumn.Format = DateTimePickerFormat.Custom;
dateTimeColumn.CustomFormat = "dd/MM/yyyy HH:mm:ss";
radGridView1.MasterTemplate.Columns.Add(dateTimeColumn);
this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

for (int i = 0; i < 10; i++)
{
    this.radGridView1.Rows.Add(DateTime.Now.AddDays(i));
}

Workaround: custom filtering: http://docs.telerik.com/devtools/winforms/gridview/filtering/custom-filtering