I have database with medical equipment. In FIELD 1 have data with period of checking (in every 1, 2, 3, 4 years). In FIELD2 have data with last (05.05.2013). And FILED, 3 where I want to calculate date when I must to check current instruments. If period is 3 years and last check is 01.01.2015, I want in F3 to calculate 01.01.2018. How can i made this calculation ?!