I thought the same thing the other week, but noticed it disappears when there's not enough space to show it? Try maximizing browser, reducing font size, etc.

I see the short date info (3 years ago) on thumbnails and the full date under the video page's title and right after the number of views.

Maybe something on YouTube got updated, and you were seeing some cached bit of code that didn't jibe right.

I had the same problem, was a size cutoff caused by me setting the scaling to 200% or something. Once I reset scaling, date is back.

Hope it is that easy for you, and it isn't a bigger problem.

*sadface* Publish date is important for educational videos especially in development!