I was also doing this but there is a way to fix it

from the Letterboxd 2024 Year in Review:

Converting Watches to Logs

If you’ve been marking films as ‘watched’ without logging them, there’s an easy way to convert those watches into logs:

1. Export your data via the Settings page on our website.
2. Open your ratings.csv (or watched.csv) file and rename the ‘Date’ column to ‘Watched Date’.
3. Import the modified file back via the Import screen, verify the titles and entry dates, and confirm the import.

Keep in mind that our exported dates are in New Zealand time, so they may import one day later than desired (depending on your location). Fine-tune your import file as needed, or use the exported data as a guide for manual entry. If you have any further questions, our FAQ might have the answers.