User can log his/her time on the scheduled tasks. Project admin can also log time on behalf of other members

To log time, go to tab Timelog

If you have scheduled tasks in the period just simply enter the time and click log to log your time.

You can also log time for task which is not scheduled by clicking on the white space, a dialog will show up so you can log working time or time off

Choose task from the dropdown box or click on + button to create a new task.

Move to Time off tab to log time off