Yes, when the original post is deleted from the server (not just the blog, but the Tumblr servers), there is no root post for notes to be added onto, and also no root post for time to be counted from, so it starts from zero. Most computer operating systems use Unix, which was launched in 1971 with t.he epoch date of midnight on January 1, 1970 as 1. Therefore zero is one second behind that date: December 31, 1969. Also, very unfortunately, this also means nobody except you and anyone you reblog it to will see this explanation, as you cannot open the notes to see comments when there are no notes.