You can use GOMONTH() function

ldDate = {^2009/10/01}
lnYears2Add = 1
ldNewDate = GOMONTH(ldDate, lnYears2Add * 12) 

>If in first text box value= oct 2009, i want value in 2nd text sep 2010 where year=1
>
>If in first text box value= oct 2009, i want value in 2nd text sep 2011 where year=2
>
>
>increase 12 month