Update post question?!

If i update a post that is 1 year old, and not change the date of the post published (keep that 1 year published) but i update the post: new videos, info etc...
Google will know to read the new info and "rank up" the post or i need 100% to post the actual date and time on post to except something?

It will take sometimes to index new post/info but i don't know which date will show.