Edit a column only once

Hello! I want to define a value on a column with an IF expression, but only once. I will explain:
I have 3 columns, the first one is a column that indicates status of a task (ie, not started, started, finished).
The second column sets the date and time when the status changes to “started” (IF([Status]=“started”;now();"")).
The third column does the same, but when the status is “finished”.
This way, I have a record of when the tasks started and when ended.
The problem is that if I do it with the IF expression, then when I change the status from “started” to “finished”, the second column goes blank (of course, as I have “” and there is no “do nothing” command on excel).
How can I solve this? is there any way to set the now() value and not erase it if the status changes? maybe with an action or workflow, or changing the app formula?
Thanks in advance.

Have you tried the ChangeTimestamp type columns?



Thank you Suvrutt. That solved my problem.