While it's less common in this kind of contexts, it's correct. Have you reported it?
Well, it was just part of the question, not a rejected answer, so I didn't report it. But is there a difference? I've always seen dzisiaj in other vocab references. When would one be preferred over the other?
When I was in Polish school the teacher always used "Dzisiaj jest...". I also would like to know what the difference is. Maybe dzisiaj is more formal?
I am a Polish native speaker and had no idea if there is a difference, so I checked
they say dziś rhymes with different words then dzisiaj. :)
Why is neither dziś nor czwartek in instrumental case with być as the verb here?
No. Instrumental is used for Y in "X is Y" sentences if both X and Y are noun phrases.
"Dziś" is an adverb, so that rule doesn't apply.