Org mode allows you to clock the time you spend on specific tasks in a project.
Start the clock on the current item (clock-in). This inserts the ‘CLOCK’ keyword together with a timestamp. When called with a C-u prefix argument, select the task from a list of recently clocked tasks.
Stop the clock (clock-out). This inserts another timestamp at the same location where the clock was last started. It also directly computes the resulting time in inserts it after the time range as ‘=>HH:MM’.
Update the effort estimate for the current clock task.
Cancel the current clock. This is useful if a clock was started by mistake, or if you ended up working on something else.
Jump to the headline of the currently clocked in task. With a C-u prefix argument, select the target task from a list of recently clocked tasks.
The l key may be used in the agenda (see The Weekly/Daily Agenda) to show which tasks have been worked on or closed during a day.