The ability to mark an item as played at an unknown date is a perennial request (as seen here, here, etc.) with the main motivation behind it being to have the item show as watched, but exclude it from your year-in-review and the play-date/time-based statistics in your all-time stats (like these).
As the play date is essential to Trakt’s design, I have a suggestion for a workaround: add an option in the settings to allow the user to set an “Ignore Before” date for their date-based stats. (Or perhaps for all stats?)
The most common use would probably be to set it to the date you joined, or just after you manually marked a bunch of old items as watched. But if you wanted to keep some older shows/movies as watched when they released, or whenever, and still exclude other items, you’d just use an earlier cutoff date and log those items before it.
This wouldn’t require any changes to the way Trakt stores play data, or any UI changes beyond adding a field to the settings page and possibly tweaking some stat displays. The main issue I could see would be communicating that this feature exists and how it works. But the stats are already VIP-only, and there’s already a “Year In Review” section in the settings where power users might look to customize their stats.
(I know it’s not quite that simple. There would be some details to work out in terms of which stats these ignored plays should or should not be counted in, and how to handle things like the all-time over-time stats e.g. episode plays per year/month/day. Then again, those over-time stats are already wrong if you’re manually logging things at arbitrary times, so those plays could be safely excluded.)
I hope this is something you can consider, or that it at least gives you another potential angle to come at the “unknown play date” issue from. Thanks!