There is no straightforward formula to get the percentage change for a stock X days back in time. However using the "price" API from Google finance we can get the price at two points in time and calculate it ourselves. The (hardly readable) inline formula would look like the following:
=(GOOGLEFINANCE("NYSE:ADM","price")-INDEX(GOOGLEFINANCE("NYSE:ADM","price",TODAY()-15),2,2))/(INDEX(GOOGLEFINANCE("NYSE:ADM","price",TODAY()-15),2,2))*100
In the above we find the difference between the current price of NYSE:ADM and subtract it with the price from 15 days ago. We then divide it by the price 15 days ago and time it by a 100 to get the difference in percent.
We can split this into two simple Google finance calls, first we get the current price for ADM (Price Today):
=(GOOGLEFINANCE("NYSE:ADM","price"))
We then get the price 15 days back (Price X Days Ago):
=INDEX(GOOGLEFINANCE("NYSE:ADM","price",TODAY()-15),2,2)
We can then write the formula as:
((Price Today - Price X Days Ago) / Price X Days Ago)*100
You can find this as a working demo here. I have made two examples, one that is inline like the above and one where I have split it across multiple lines for it to be eassier to read.
That is all there is to it, I hope you found this helpful!