This is a topic of never ending discussions among many cachers and communities, but the rules do not specify that precisely. I believe we should use there a common sense here, which would be logging the date when the cache (the place) was visited. When you write the log, the link you are clicking is
Log your visit
and not "log your research results" or something similar.
This is quite obvious in the other cache types (physical log in cache/event logbook), but I don't see a reason why to do it other way for these "virtual caches" (earth, virtual, ...). Basically, if you are recommended to add a photo from that place, the log should be the date this photo was taken.