You can programmatically set the tooltip to anything that you can programmatically access.
I'm not sure it's good programming practice though. The tool tip takes some time to appear.
In the following example I'm creating some dataset to populate a datagridview. I didn't use datetimes, but just composed the tooltip for the first column of the sum of the strings in columns 2 and 3.
DataTable dt = new DataTable();
private void button1_Click(object sender, EventArgs e)
{
//put some data into a table
int[,] time = new int[,] { { 0, 4, 1 }, { 1, 5, 2 }, { 15, 10, 3 } };
dt.Columns.Add("x");
dt.Columns.Add("y");
dt.Columns.Add("z");
for (int i = 0; i < 3; i++)
{
DataRow dr = dt.NewRow();
dr[0] = time[i, 0];
dr[1] = time[i, 1];
dr[2] = time[i, 2];
dt.Rows.Add(dr);
}
dataGridView1.DataSource = dt;
}
private void dataGridView1_CellEnter(object sender, DataGridViewCellEventArgs e)
{
dataGridView1.ShowCellToolTips = true;
Point loc = dataGridView1.CurrentCellAddress;
if (loc.X == 0)
{
dataGridView1.CurrentCell.ToolTipText = String.Format("{0} ",
dt.Rows[loc.Y][loc.X + 1].ToString() + dt.Rows[loc.Y][loc.X + 2].ToString());
}
}