You need to use
Dim a as long = DateTime.TimeParse(EditText1.Text)
Dim b as long = DateTime.TimeParse(EditText3.Text)
With this method you get ticks, so after calculating the difference b-a you need to convert it to time :
EditText3 = DateTime.Time(b-a)
Dim a As Long = DateTime.TimeParse(edittext1.Text)
Dim b As Long = DateTime.TimeParse(edittext2.Text)
Dim p As Period = DateUtils.PeriodBetween(a,b)
EditText3.Text = p.Hours&" hours, "&p.Minutes&" minutes"