To reproduce:
- Use this code:
private void Form1_Load(object sender, EventArgs e)
{
    CenterToParent();

    radGridView1.CellEditorInitialized += radGridView1_CellEditorInitialized;

    GridViewTextBoxColumn columnName = new GridViewTextBoxColumn();
    columnName.Name = "Name";
    columnName.HeaderText = "Name";
    columnName.FieldName = "Name";
    columnName.MaxLength = 50;
    columnName.Width = 200;
    columnName.TextAlignment = ContentAlignment.MiddleLeft;
    radGridView1.MasterTemplate.Columns.Add(columnName);

    GridViewDateTimeColumn columnDate = new GridViewDateTimeColumn();
    columnDate.Name = "Date";
    columnDate.HeaderText = "Date";
    columnDate.FieldName = "Date";
    columnDate.Format = DateTimePickerFormat.Custom;
    columnDate.CustomFormat = "MM/dd/yyyy";
    columnDate.FormatString = "{0:MM/dd/yyyy}";
    columnDate.NullValue = null;
    columnDate.Width = 100;
    this.radGridView1.Columns.Add(columnDate);

    radGridView1.Rows.Add("Row 1", null);
    radGridView1.Rows.Add("Row 2", null);
    radGridView1.Rows.Add("Row 3", null);
    radGridView1.Rows.Add("Row 4", null);
}

void radGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
{
    if (e.ActiveEditor is RadDateTimeEditor)
    {
        ((RadDateTimePickerElement)((RadDateTimeEditor)e.ActiveEditor).EditorElement).TextBoxElement.MaskType = MaskType.FreeFormDateTime;
      
    }
}

- Start the application and star edition date time value. You will notice that the editor value is initialized incorrectly. This occurs only when the cell is edited for the firs time.

Workaround:
void radGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
{
    if (e.ActiveEditor is RadDateTimeEditor)
    {
        ((RadDateTimePickerElement)((RadDateTimeEditor)e.ActiveEditor).EditorElement).TextBoxElement.MaskType = MaskType.FreeFormDateTime;
        e.ActiveEditor.Value = radGridView1.CurrentCell.Value;
    }
}