Tracking time for Asana tasks is easy & accurate using Time Doctor (https://www.timedoctor.com/). This video shows how to use Time Doctor to track the amount of time that you and/or your employees spend working on Asana tasks.
Time Doctor integrates & syncs with Asana and provides you with reports on how much time has been spent on each Asana task and project, but also additional detail on how users spent their time. For example, you can see which websites and applications were used while working, schedule work hours for your employees, and much more.