I'm building a database to keep a record of medicine I've administered to my flock of sheep. I have a table for all the animals on my holding, a table for medicines I currently hold and a table for medicine application. The medicine application table has related fields back to the Animal ID and the Medicine ID. Within the Medicines currently held table there is a withdrawal period in days, I would like to use that number of days in the medicine application table to calculate the when the withdrawal period ends based on the application date. I can't seem to find an easy way to achieve this, other than manually looking up the withdrawal period and adding it to the application date, and selecting the date manually.