We can use the org clock to track the time it takes to do different things.

We can clock in and out of a task (or any item):

(org-clock-in)
(org-clock-out)

There is only 1 clock in org so clocking in on another task clocks you out of the previous task.

Changing a task that you've clocked in to DONE state automatically clocks it out.

If you are using lexial binding it must be the first line of the file.