suitable macro

I need assistance in compiling a macro to determine when my medication should be re-ordered.
Unfortunatly my(several)types of pill are supplied in different standard quantities.
I was hoping to be able to associate the starting quantity to the date purchased, the other variable being quantity per day and the restocking period (say 7 days notice)
Could a reminder automatically appear on the desktop when this date is reached?
Or maybe I am asking too much?