service interval

if  I have 250 units and there are 10 unit of them its paper will be renew in 26-9-2019 . and  10 unit will be renew in 15-1-20 and other units will be renew in different date then when i want to add service interval to those units . i must calculate for each unit the period that it need so why you can simplify this process by giving customers a calendar to add this event that he need with out any calculations . he will only select the date form calendar

thanks