Web Forms :: Display Date In TextBox From Day - Month - Year DropDownList

I have 3 drop down lists for days,months and years,also i have a textbox to show the selected values from the 3 drop down lists when i select a month it shows in textbox and if i select another month it shows in textbox beside the first selected month.i want not to repeat the values in textbox.

the code:

protected void ddl_day_from_SelectedIndexChanged(object sender, EventArgs e)
{
txt_exhib_dateFrom_add.Text ="";
day_from = ddl_day_from.SelectedItem.Value;
txt_exhib_dateFrom_add.Text = day_from +"/";
}

[Code] ......