From the products page at https://buy.garmin.com/en-US/US/into-sports/cycling/edge-520/prod166370.html
LiveTrack is Getting Easier to Use
LiveTrack lets family and friends track your location in real-time while you’re riding, and now it’s even easier to use. When you enable auto-start, you can spend more time focusing on the ride and less on the technology. LiveTrack will automatically detect when you start riding, which means there’s no need to even reach for your phone anymore. Auto-start is also compatible with the new Strava Beacon, which lets Strava Premium subscribers take advantage of the same great feature. It’s never been easier for you and your loved ones to have peace of mind while you ride.
However, my Edge 520 (v7.20) does not have the setting to automatically start the timer, the 820 does. From the manual:
http://www8.garmin.com/manuals/webhelp/edge820/EN-US/GUID-F2A490BD-1D4B-4BB8-97C5-90B55733092F.html
Does this mean that this will be added in a future firmware update for the 520 or is the products page just mistaken?