Hi,
your
ComboBox3.SelectedIndex = i
is not doing anything useful.
Your
Dim parsedate As DateTime = ComboBox2.SelectedItem.ToString
will give the same result on each iteration of your loop, and all items in ComboBox3 will end up showing the same number of days, which depends on which ComboBox2 item was selected when you clicked the button.
So my guess is you wanted the former to be
ComboBox2.SelectedIndex = i
That would result in varying strings in ComboBox3.
But then I'm puzzled why you are using SelectedIndex and SelectedItem at all. A simple
Dim parsedate As DateTime = ComboBox2.Items[i].ToString
should suffice.
:)