I had this same error for a while - it turns out the error is a "phantom error" and really isn't about the date, at least in many cases. I fixed a completely unrelated error relating to value of attribute "preserveAspectRatio" is invalid. I fixed that in my titlepage.xhmtl. The minute I did, the date error went away. So if you're getting a date error, and think the date looks good, make sure with Sigil that you don't have other errors. It could be the date is fine!
I have all my notes on my LisaShea.com website and am happy to answer questions.