I take a different approach.  When I have something I want to share privately, I post it to Vimeo.  They have good privacy controls and it's less well known so the chance of someone stumbling into it is smaller.  When you're ready to go public, post to youtube and make the Vimeo one public as well.  If you do your SEO and setup your profile with a link, you'll get two good backlinks to your site.

GOOD question, but I'd suspect it would be the ORIGINAL date.

Debe Maxwell, CRS I found this answer in YouTube Help: 

You cannot change the publish date. You can schedule a video to appear published on a specific date during the upload process.

Changing a video that has already been published to be scheduled to a different date will still retain its original upload date.

Ryan Huggins - Thousand Oaks, CA has a great idea.

Good question!!

Not knowing anything about this I say Kathleen Daniels, Probate & Trust Estate Specialist has your answer

I believe that the date stays the same, however it would be worth experimenting with.

I do believe this is over my pay grade, Debe! 🐱