I just discovered this, while editing an existing blog:
When you edit an existing blog, and Save as Draft while you are doing that, it will get a new published date (current date) when you Save it again.
My workflow is to edit an existing blog (usually to correct a typo, add a link to the next post, or to rephrase a fuzzy sentence) and then Save it again.
In that case, the publish date remains the original one.
I used the Save as Draft approach this time, because I wanted to restructure a sloppy post that I wrote in a hurry.
I Saved as Draft, then used the View option to see if it rendered the way I wanted it.
When I Saved the completed work, I saw that the date was updated.
