Our recommendation is to create a new calendar for your time recordings. You could technically use your personal calendar, but once you want to share your time recordings with somebody else, it would be a lot of work to separate all the entries.
Within that calendar you can create new entries representing your work hours. Every entry must have a hashtag (#) that identifies the group or project of the work-package. Behind that tag you can leave a comment to document the work you have done.
For example:
#Project42 Research
Make sure that the entries start-time and end-time matches your actual time consumption. Once you create a new report, all entries of your calendar that have a #tag in their title will be used to create a report. You are also able to apply filters.