While managers could always edit existing time details via Reports, it wasn't possible to add new time entries for an employee or change the employee that time entry was associated to. As of today, that's all possible!
Firstly, managers can edit any time entry and assign it to a different user. Simply click on an editable entry, and click the employee's avatar to re-assign it.
Next, on the Time tab, managers can now click to swap to the timesheet of any employee.
Finally, to add time as one of your employees, simply click 'Dock' next to their name, and your normal Dock will add time on their behalf (instead of yours). This makes it really easy to enter a bunch of time at once just using the keyboard.