data grid views are dynamic controls used to render a variety of data. cells some how represent disparate data types, and therefor use a "Format" string to call datum's ToString() method passing in this format string.

If you wish to restrict what is entered into a cell, handle the CellValidating and or CellEndEdit events:

http://msdn.microsoft.com/en-us/library/system.windows.forms.datagridview.cellvalidating%28v=VS.100%29.aspx

http://msdn.microsoft.com/en-us/library/system.windows.forms.datagridview.cellendedit%28v=VS.100%29.aspx

When DataGrid is populated set the format of the column, so this code comes into my Form1_Load event.....
dataGridView1.Columns["Rel_Date"].DefaultCellStyle.Format = "dd MMM yyyy";


In DataGridView's CellValidating event use this code:

DateTime temp;
                if (dataGridView1.Rows[e.RowIndex].IsNewRow)
                    return;
                string format = "dd MMM yyyy";
                if (!DateTime.TryParseExact(e.FormattedValue.ToString(), format, null,0, out temp))
                {
                    e.Cancel = true;
                    dataGridView1.Rows[e.RowIndex].ErrorText = "The date entered should be in \"dd MMM yyyy\" format only.";
                }