In 2008, Taxpayer Anne began diverting checks received as part of her bakery business. Anne does not deposit these checks in the business's corporate accounts. Instead, she uses a check cashing store to cash the payments and hides the cash under her mattress. Anne does not tell her accountant about these payments. As such, they are not reported on either her corporate or personal returns. From 2008 through 2015, except for 2011, Anne's individual tax returns omitted the income associated with these check payments. Anne timely filed her returns in each respective year on April 15. From 2015 forward, Anne began filing complete and accurate returns.

On what date can Anne relax and not worry about being criminally prosecuted? What is the last date she could be assessed additional tax for years 2008-2015?