Check out MSDN Reference:
Controls contained in a TabPage are not created until the tab page is shown, and any data bindings in these controls are not activated until the tab page is shown.
In form load you can select your tab as well.
private void Form1_Load(object sender, EventArgs e)
{
tabControl1.SelectedTab = tabPage2;
dateTimePicker1.Text = "2014-05-14 00:00:00.000";
}