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, hovering over the heading for a given date will make the 'New+' button appear. Click this to create a blank time entry under that date.

You can click into the time entry to edit it - add time, assign contact/project/task labels, and take any notes for the time entry. When done, click the tick on the right-hand side to save your changes.