Multiple Time Stamps depending on choice in Drop Down List Choice

Hi everyone,

My very first post here but I've found the help in here to be so good I just knew where to turn now that I can't seem to solve it on my own.

I'm trying to put together a sheet that tracks task completion. Basically, every row represents a task. In column L I have created a drop down where the CR agent (this is for a callcenter) chooses the status of the task he/she is working on (the options being Standing, In Process, Pending, Closed). In the following four columns (M, N, O and P) I would like to see the time and date (i.e. command NOW) for when each of these stages where started.

So, if an agent chooses "Standing" in the dropdown (column L), then a timestamp should be added to column M. When the agent later updates the dropdown (column L again) to status "In Process", then that adds a new timestamp in column N (please note that none of the timestamps should ever change).

In similar fashion, changing the dropdown status (column L still) to "Pending" should add a timestamp in column O and changing it to "Closed" should add a timestamp in column P.

I should be able to figure this out on my own, but I'm just giving up Anyone help please?

Thanks!!