The way I do it : On Form1 I listen for the Form2.FormClosed event like this

Form2 myForm2;
private void textBox1_TextChanged(object sender, EventArgs e)
{
myForm2 = new Form2();
myForm2.FormClosed += new FormClosedEventHandler(myForm2_FormClosed);
myForm2.Show();
}

Then when the event happens :

void myForm2_FormClosed(object sender, FormClosedEventArgs e)
{
this.TextBox1.Text = myForm2.TheDate;
}

On Form2 you need to have a Public property TheDate which you set when they click on your calendar OnOkClick you just close the form. hope that makes sense.

I agree with KarstenK it boils down to: if(A!=B) { if(A) {//do stuff} else {// do something else} }