Those know-it-alls at Big Calendar will tell you the official start of fall is Sept. 22 or Sept. 23, depending on the year. But when does it actually start to feel like autumn where you live? Time has created an online tool to answer that exact question, studying 21 years worth of data from the National Oceanic and Atmospheric Administration. You can enter your city or county to find out exactly when you should break out the sweaters and apple cider. For example, fall has yet to arrive in New York City, Chicago, Boston, or Dallas. But autumn came to Los Angeles on Sept. 4, where a seasonally appropriate flannel might seem a bit warm as temperatures once again approach 90. Ah, fall. (More autumn stories.)