My phone's just reached its 20% battery warning for the day so I checked my usage to see how I did today. It says 3:43 usage and 15:21 standby. Seems pretty good, except I pulled it off the charger about 15 hours ago. So how does it figure the usage number? Is it time since the charging was completed, which would be pretty close by my estimates? Or is the standby time the more accurate number and the usage is part of that? I thought the two numbers summed together was time since unplugging the charger, but that doesn't add up.