Dates Values In A Dropdown List Box

I have 3 drop down list boxes - One for month, day and year. I'm trying to get the value of what the user selects on those drop down list boxes. I have dynamically populated my drop down list boxes and I'm trying to output the end value on a label

private int month;
private int day;
private int year;
protected void Page_Load(object sender, EventArgs e)
{
for (month = 0; month <= 12; month++)
{
DropDownList2.Items.Add(month.ToString());
}
for (day = 0; day <= 31; day++)
{
DropDownList3.Items.Add(day.ToString());
}
for (year = DateTime.Now.Year; year <= DateTime.Now.Year + 75; year++)
{
DropDownList4.Items.Add(year.ToString());
}
}
protected void Button4_Click(object sender, EventArgs e)
{
month = ?
day = ?
year = ?
Label7.Text = month + "/" + day + "/" + year;

The problem right now is I can't figure out how to actually retrieve the value the user uses