You can't do that with ordinary TextBox, but in LostFocus You must change the Value of the Text2 to whatever You want:
Text2->LostFocus event
this.Value = DATE(YEAR(thisform.Text1.Value)-1, MONTH(this.Value), DAY(this.Value))
this.Refresh()
Against Stupidity the Gods themselves Contend in Vain - Johann Christoph Friedrich von Schiller
The only thing normal about database guys is their tables.