{date} is the date the Calibre entry was made. It has little bearing on the date the book was {published} (or first published).
I prefer to keep the {date} as it was to know when
the Library entry was created. I use a custom column {#edited} to record the date that I touch up the book and verify the code. YMMV
If you want to 'fill in the blanks', You could use Bulk metadata edit: Search and replace: (REGEX mode)
Code:
Search: {date}
For pattern: (*+)
Replace: {published}
with value: \1