How do I get two forms to talk to each other?

i am working on a library system. i have to calculate fine on receive book form... due date is after 14 days of issue date, which i calculated on issue book form.. if the the book is return after due date there will be fine of 10 rs per day .. i am unable to calculate it.. 

there is a form for issuing books. i have calculate due date and issue date here.

private void Form17_Load(object sender, EventArgs e)
        {
            textBox1.ReadOnly = true;
            DateTime d = DateTime.Now;
            textBox1.Text = d.Day + "/" + d.Month + "/" + d.Year;


            textBox2.ReadOnly = true;
            DateTime today = DateTime.Now;
            DateTime dueDate = today.AddDays(14);
            textBox2.Text = dueDate.Day + "/" + dueDate.Month + "/" + dueDate.Year;

now there is another form for receiving book. there fine will be calculated on above criteria after clicking submit button. i want to get dueDate from Prev form... 

or if i get the dueDate from database via oleDb DataAdapter, how to get the value that is stored in data set and assign it to a variable.

OleDbDataAdapter da = new OleDbDataAdapter(" select dueDate from issue  where  title = '" + comboBox1.SelectedItem + "'  ", con);

               DataSet ds = new DataSet();
              
               da.Fill(ds, "issue");