Admins can track time on behalf of other users.
You can do this from the time tab. Just below the dock, click on My Time and a dropdown of the users on your account will pop up.
Select the user you want to track time for, and their time entries will be displayed.
To track time as them, hover over one of the days of the week and click 'dock'. Anything you type in the dock now will be logged as them. If you do not click dock first, it will simply log the entry as you.