This has come up a lot in the past, and I want to add my 2 cents.
Time tracking in Asana is the key feature we miss. We use the free version of Asana which provides tons of useful features, but we pay everhour a ridiculous amount of money to do time tracking only. We would much rather have that money go towards Asana. If time tracking was implemented in Asana we would upgrade to premium on the spot.
Asana funnels so much money into time-tracking products. Why not implement your own? Users has been screaming for native time tracking for years.