Many people complain about the fact that Google has a record of where you go if you opt in to location tracking, but people should be able to extract this record to get an automated working time estimate without needing to do do anything to prepare ahead of time, if they have an Android phone and haven't disabled the tracking.
I'm not sure about the completeness/time accuracy of the logged data, but maybe you could weigh in on the presumed legal validity of such an approach?
It's entirely possible that is less accurate for others, but it's a source of data that many people in this situation should already have, so you can use it for those for whom it is more accurate, and ignore it for others.
However, it is geared towards protecting the employer from wage theft by employees in the form of late clock-in, early clock-out, etc. Eg, when an employee inflates their time worked.
if time-tracking somehow solves the bare moneys/hour problem, other, perhaps worse, conditions could surface (thinking of what it's like to work in an online shop warehouse here, in particular, where time-tracking is .. not a complete "solution.")
traditionally, afaik, unionization is the way out of situations where a great number of workers are disadvantaged by some policy set by a few people in the industry.
this is a social [(science)?] problem, not a technological one.