Since I read in one of your comments that you use Trello, I can weigh in here. I manage a few teams working in Scrum and Kanban on small-ish projects, and I find Trello great for this, so I stick with it.
While, on the front, Trello's usability is super-simple and intuitive, don't think that it isn't a more powerful beast underneath. Any change to any card in Trello has a very thorough and in-depth history that can be accessed through their API. What it handles is a long list, but highlights that may help you include:
- Dates and times cards are moved from one list/board to another
- What tags are assigned to a card and when were they assigned
- Who is assigned and when were they assigned
- Etc. etc.
So, if your tasks were completed 6 months ago (and I guess there's some indicator e.g. the card is in the "completed" column, or archived), you can query this historical information from Trello and work backwards.
For my projects and teams, I've written a simple website that gathers this information on user stories, when they were added, who was assigned, when it was moved from the backlog, into progress, testing and complete and from that, generate the burndown, lead time and cycle time, and graph it all on the fly. So, while Trello is simple to use, it can be pretty powerful as well.