The first formula gets applied to newly created records, the second on edits (both Edit view and inline edits). You can access the currently typed text in the formula, so you can use it to transform what was entered.
Then you just type something in the field:
… and it gets immediately processed by your calculated field formula, storing the results:
You can do basically everything; and it’s extensible.
I’ve just been browsing the web to have a look at how SugarCRM and SalesForce do formulas and calculated fields these days, and unless I am very wrong (I admit to being biased on this one ) these Twig-based fields can be so much more powerful, extensible and elegant.
We have filters, inheritance, includes, blocks, extensibility, sandboxed security, …
You need a way to trigger the beginning and end of when the time is counted - were you thinking of making that a manual gesture, or automatic (might be difficult…)?
PowerFields will let you do things on fields, when they are created or edited. PowerWorkflows will let you do things when a button is pressed in a view. I also see a possibility to use the tracker table which will record when users enter each view, which might be valuable information for the start of the timer.
With these elements, and a bit of extra PHP code to glue it all together, you might be able to achieve something: an approximate, not entirely accurate, timing.