Changing watched date to unknown leaves 'ghost' watch date

It appears that if you edit an existing watch date to unknown, the original date is retained for sorting purposes.

Steps to Reproduce

1. Add a movie with one watched date to a list with other watched titles.

2. Sort the list by last watched date; observe the movie is in the correct position

3. Edit the watched date of the movie to 'unknown'

4. Refresh the list; observe the movie incorrectly retains its sort position

5. Remove all watched history from the movie

6. Mark movie as watched with unknown date

7. Refresh the list; observe the movie correctly appears at one end or the other of the list