Hubstaff is a time-tracking and employee monitoring solution available for Windows, Mac, Linux, Chromebook, iOS, and Android.
Hubstaff's Asana time-tracking integration allows employees to track time on their Asana tasks. When they're done working on a task, they can mark it as completed from Hubstaff and the time will be posted to the Asana task.
Integrating Hubstaff and Asana only takes a few minutes and after that there are automatic updates (project teams stay synced). The Hubstaff reporting area is a nice way to see the exact time spent working on project tasks.