If today I were to go into Letterboxd and log a movie as having been watched a year ago, there will be two entries that show up in my exported Letterboxd data. One in watched.csv, with today's date associated with it, and one in diary.csv with the actual watch date associated with it.

Currently Simkl is marking imported movies with the watched.csv dates only, even in cases where more accurate dates for items can be found in diary.csv. It should use dates from diary.csv instead where available.

Note that it is possible to mark a movie as watched in Letterboxd without recording the specific date. In this case there will be no diary.csv entry, and it would be necessary to fallback to using the watched.csv entry as an indicator that the movie has been watched. In that case, using the date added as the watched date would probably be the best guess.

Also note that unlike watched.csv, diary.csv may have multiple entries for the same movie if it was logged for multiple watches on different dates.