This is a great addition — I’ve been looking forward to this! 
Being able to mark items as watched without a specific date is super helpful for backfilling older history.
I’d like to suggest an expansion of this feature, though: allowing multiple unknown-date watches. Currently, it only allows one “unknown date” entry, but what if I watched something multiple times before joining Trakt, or rewatched it later and forgot to track it? Sure, sometimes it’s fine to assign an approximate date, but I’d much rather be able to add another “unknown date” play.
Expanding this would make it much easier to accurately reflect total watch counts without resorting to fake dates for multiple “unknown” plays.
Thanks for continuing to improve Trakt!