Script calculator to display under a post.

I need a simple calculator to install just under home page first post. This calculator should calculate fertile period on ogino knauss method, thats needs two box, one with calendar selection tent beside, for date of last mestruation and one for avarage normal length of mestruation. The script have to do the simple operation: avarage length of mestruation - 18=first fertile day avarage length of mestruation - 11= last day fertile and give the result displayed as date, based on last mestruation. exemple: last mestruation 1 january 2012/ avarage days 28 28-18=10 28-11=17 first fertile day 10 january 2012/ last day 17 january 2012. This web site is for the italian market, the calculator should have arrow frame and an italian title "Calcolare periodo fertile". Have to be also installed. I upload for you a schematic picture. Note the text boxes have to be blank but over the first should be write " Inserisci Data Inizio Ultimo Ciclo" and the second " Giorni Durata Media ciclo". At work done I'll give you details to enter wordpress blog account for install. Calendar best in italian.