Not „Dziś to długi dzień”?
dziś is an adverb.
And „to” cannot be used with adverbs?
To is not a verb, while jest is a form of the verb, być (to be.)
this was very confusing. The English sentence should read: "Today is a long day" for this exact translation...not "It is a long day today". I would never have come up with this Polish translation with the way the English sentence was written.
Agreed, I also think "Today is a long day" is better, I made it the default answer.