So I just wanna share the answer to my question. So this is the code:
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
Dim NextTime As Date = Now
TextBox1.Text = NextTime.AddDays(set a count here.0) ' for example (100.0)
End Sub
This can be use for due dates or date counter.