Need To Track Time Spent Active In Workbook

My team and I are attempting to track how much time we are spending in each Excel workbook we create (all workbooks are created from a single template). The problem is that we are often jumping back and forth between different workbooks throughout the day so manual entry methods are neither accurate or practical. Is there a way, with macros or formulas, to embed a time tracker in the template workbook that would only record active (when workbook is selected) time in the workbook?? Thanks

Hey, this is exactly what I am looking for! However I am having some trouble implementing it. Should it be as easy as adding a 'Log' sheet and pasting this code into the ThisWorkbook module? When I do this I get a timestamp added to column A only when I close then reopen the workbook. Thanks for your help

This is great! Would be great if you could help to improve few things:
1. When you swich to any other app, for example internet explorer this should meen that the workbook is deactivated. Now it looks like that the book is deactivated only when you close excel or swich to other workbook
2. How to add user name to the logg, as several users can work with the same file, and it would be great to know how much time each user has spent
3. How to make this macro add: log of file name, user name, time spent to the file save on network, so it would collect all info from all users to the one file stored on network?
Thank you!