Add textbox default time and datagrid time

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C#2.0

C#

Hi..
I want to calculate between textbox value and datagrid time

textbox time = 17 minutes
datagrid columns -starting time,finishing time

default time =17 minues
staring time=7.59
finishing time = default time + starting time
17 + 7.59 =8:16

What I have tried:

txtCuringTime.Text="17";

private void DgASideView_CellValidated(object sender, DataGridViewCellEventArgs e)
{
if (DgASideView.CurrentCell.ColumnIndex == 1)
{
DgASideView.Rows[DgASideView.CurrentCell.RowIndex].Cells[1].Value =
cfun.convertdecimal(DgASideView.Rows[DgASideView.CurrentCell.RowIndex].Cells[0].Value.ToString()) + cfun.convertdecimal(txtCuringTime.Text.ToString());

}
}