Formula for timing duration of a section of a survey

I have researched this forum and others but could not find a solution yet..

1. What is the problem? Be very detailed.
I need to be able to calculate how much time the enumerator is taking to fill a specific section of a survey. I understand that the function for that is "timestamp".

2. What app or server are you using and on what device and operating system? Include version numbers.
I am using ODK Collect v1.23.3 on a Samsung Galaxy A5 2015.

3. What you have you tried to fix the problem?
I tried the following formula in my Excel spreadsheet:
Type: calculate
name: start_timestamp
calculation: once(now())

then I have a number of questions (this is the section I would like to time

type: acknowledge
name: finished
label: finished?

Type: calculate
name: end_timestamp
calculation: if(${finished}!='',once(now()),'')

Type: note
name: started_at_start_timestamp
label: started at ${start_timestamp}

Type: note
name: finished_at_end_timestamp
label: finished at ${end_timestamp}

4. What steps can we take to reproduce the problem?

5. Anything else we should know or have? If you have a test form or screenshots or logs, attach below.

Thank you very much!

Have a look at this post (in Kobo forum as it happens); it sounds very similar and may also address your need: